TELT225- Cisco 1- Introduction to Networks- Spring 2024-2025 (2)

Network Layer

Network Layer Functions

• Addressing

• Encapsulation/Decapsulation

• Routing

Network Layer Protocols

• IPv4

• IPv6

• IPX

• AppleTalk

• CLNS/DECNet

• Internet Protocol (IPv4 and IPv6) is the most widely-used Layer 3 data carrying protocol

IPv4

• Low overhead.

• Provides necessary functions to deliver a packet from source to destination.

• Basic characteristics:

  • Connectionless - No connection established before sending packets.

  • Best Effort (unreliable) - No guarantee of packet delivery.

  • Media Independent - Operates independently of the medium carrying the data.

IPv4 Packet Header

• Includes Version, IHL, Type of Service, Packet Length, Identification, Flags, Fragment Offset, Time to Live, Protocol, Header Checksum, Source Address, Destination Address, Options, and Padding.

Time-to-Live (TTL)

• Decreased by at least one each time the packet is processed by a router (hop).

• When the value becomes zero, the router discards or drops the packet.

• Prevents packets from being forwarded indefinitely between routers in a routing loop.

Protocol

• Indicates the data payload type that the packet carries.

• Enables the Network layer to pass the data to the appropriate upper-layer protocol (e.g., 06 TCP, 17 UDP).

Type-of-Service

• Used to determine the priority of each packet.

• Enables a Quality-of-Service (QoS) mechanism to be applied to high priority packets such as voice packets.

Fragment Offset

• IPv4 packet uses the Fragment Offset field and the MF flag in the IP header to reconstruct the packet when it arrives at the destination host.

More Fragments flag (MF)

• A single bit in the Flag field.

• When set, it means that it is not the last fragment of a packet.

• An unfragmented packet has all zero fragmentation information (MF = 0, fragment offset =0).

Don't Fragment flag (DF)

• When it is set, then fragmentation of this packet is NOT permitted.

• If a router needs to fragment a packet but the DF bit is set to 1, then the router will discard this packet.

IPv4 Header Fields

• Version - Indicates the IP version number (4).

• Header Length (IHL) - Specifies the size of the packet header in 32 bit words (4 bytes). If IHL= 5 à Header = 5*4 = 20 bytes (the minimum size).

• Packet Length – indicates the entire packet size, including header and data, in bytes.

• Identification - used for uniquely identifying fragments of an original IP packet.

• Header Checksum - used for error checking the packet header.

• Options - There is provision for additional fields in the IPv4 header to provide other services but these are rarely used.

Dividing Networks

• Large networks are separated into smaller networks that are interconnected.

• These smaller networks are often called Subnetworks or Subnets.

Advantages

• Improving Performance: Reduces the traffic across the internetworks.

• Security: Securing communications and data from unauthorized access.

• Address Management: Reduces the unnecessary overhead of all hosts needing to know all addresses.

Hierarchical Addressing

• IP addresses are hierarchical: Network and Host.

• The network portion of the address for all hosts in a network is the same.

• To divide a network, the network portion of the address is extended to use bits from the host portion of the address.

• The number of bits of an address used as the network portion is called the prefix length.

• In the devices in an IPv4 network, a separate 32-bit number called a subnet mask indicates the prefix.

Communicating Between Networks

• When a host needs to communicate with another network, an intermediary device, or router, acts as a gateway to the other network.

• As a part of its configuration, a host has a default gateway address defined.

• This gateway address is the address of a router interface that is connected to the same network as the host.

• The router also needs a route that defines where to forward the packet next. This is called the next-hop address.

• Routers forward packets between networks by referring to the Network part of the address.

Routing

• A router makes a forwarding decision for each packet arriving at the gateway interface. This forwarding process is called routing.

• To forward a packet to the destination, the router requires a route to that network. If a route to a destination network does not exist, the packet cannot be forwarded.

• The routing table stores information about connected and remote networks.

• Remote networks can be manually configured on the router by the network administrator or learned automatically using dynamic routing protocols.

Routing Table Features

• Destination network

• Next-hop

• Metric

• If there are two or more possible routes to the same destination, the metric is used to decide which route appears on the routing table.

• The router may also use a default route to forward the packet.

• The default route is used when the destination network is not represented by any other route in the routing table.

Packet Forwarding

• Routing is done packet-by-packet and hop-by-hop.

• Each packet is treated independently in each router along the path.

• At each hop, the router examines the destination IP address for each packet and then checks the routing table for forwarding information.

• The router will:

  • Forward it to the next-hop router

  • Forward it to the destination host

  • Drop it

• A router processes the packet at the Network layer. So it will decapsulate the packets to get the Layer3 information and then re-encapsulate it by the Layer 2 protocol before forwarding it out the interface.

Static Routing

• Routes are manually entered into the router.

• Changes must be manually updated on every router.

• Routing information may be incomplete or inaccurate, resulting in packet delays and possible packet loss.

• Static routing does not produce any network overhead and places entries directly into the routing table with no processing required by the router.

• Configuration: Router(config)# ip route <destination network ip> <subnet mask> <exit-interface> or Router(config)# ip route <destination network ip> <subnet mask> <next-hop address>

Dynamic Routing

• Routing protocols are the set of rules by which routers dynamically share their routing information.

• When a router receives information about new or changed routes, it updates its routing table and, in turn, passes the information to other routers.

• Common routing protocols are: Routing Information Protocol (RIP), Enhanced Interior Gateway Protocol (EIGRP), Open Shortest Path First (OSPF).

• The exchange of route information adds overhead that consumes network bandwidth.

• In many internetworks, a combination of static, dynamic, and default routes are used to provide the necessary routes.

• Configuring RIP:

  • Router(config)# router rip

  • Router (config-router)#network <ip-address of the directly connected network>

Host Routing Table

• Hosts also require a local routing table.

• This table typically contains its direct connection or connections to the network and its own default route to the gateway.

• The routing table of a computer host can be examined at the command line by issuing the “netstat –r”, “route”, or “route PRINT” commands.

• You may want to indicate more specific routes from a host.

• You can use the following options for the route command to modify the routing table contents:

  • route ADD

  • route DELETE

  • route CHANGE

Default Route

• A router can be configured to have a default route.

• A default route is a route that will match all destination networks.

• In IPv4 networks, the address 0.0.0.0 (quad 0) is used for this purpose.

• The default route is used to forward packets for which there is no entry in the routing table for the destination network.

• Packets with a destination network address that does not match a more specific route in the routing table are forwarded to the next-hop router associated with the default route.

• Configuration: Router(config)# ip route 0.0.0.0 0.0.0.0 <exit-interface> or Router(config)# ip route 0.0.0.0 0.0.0.0 <next-hop address>

Address Resolution Protocol (ARP)

• ARP sends a Layer 2 broadcast to all devices on the Ethernet LAN.

• The frame contains an ARP request packet with the IP address of the destination host.

• The node that identifies the IP address as its own replies by an ARP reply to the sender.

• This response is then used to make a new entry (new row) in the ARP table called the ARP cache.

• Each entry has a pair of values: an IP Address and a MAC address.

• ARP table is stored in RAM and is maintained dynamically.

• Static map entries can be entered in an ARP table, but this is rarely done.

• If ARP fails, upper layers of the intermediary device, a router, may choose to respond to the source host with an error in an ICMPv4 packet.

• If the destination IPv4 host is on the local network, the frame will use the MAC address of this device as the destination MAC address.

• If the destination IPv4 host is not on the local network, the source node needs to deliver the frame to the router interface that is the gateway or next hop used to reach that destination.

• If the gateway entry is not in the table, the normal ARP process will send an ARP request to retrieve the MAC address associated with the IP address of the router interface.

ARP Spoofing

• ARP spoofing, or ARP poisoning, is a technique used by an attacker to inject the wrong MAC address association into a network by issuing fake ARP requests.

• An attacker forges the MAC address of a device and then frames can be sent to the wrong destination.

• Manually configuring static ARP associations is one way to prevent ARP spoofing.

• Authorized MAC addresses can be configured on some network devices to restrict network access to only those devices listed.

Proxy ARP

• Sometimes a router interface may use a proxy ARP to respond on behalf of remote hosts.

• This means that the ARP cache of the requesting device will contain the MAC address of the gateway mapped to any IP addresses not on the local network.

• Using proxy ARP, a router interface acts as if it is the host with the IPv4 address requested by the ARP request.

• By "faking" its identity, the router accepts responsibility for routing packets to the "real" destination.

Configure an IPv4 Router Interface

• To be available, a router interface must be:

  • Configured with an IP address and a subnet mask: (config-if)#ip address ip-address subnet-mask

  • Activated - By default, router interfaces are not activated (shutdown). To enable an interface, it must be activated using the no shutdown command.

• Optionally, the interface could also be configured with a short description of up to 240 characters.

Configure an IPv4 loopback interface

• A loopback interface is a logical interface internal to the router.

• It is not assigned to a physical port and can therefore never be connected to any other device.

• It is considered a software interface that is automatically placed in an UP state.

• The loopback interface is useful in testing and managing a Cisco IOS.

• Configuration:

  • Router(config)# interface loopback number

  • Router(config-if)# ip address ip-address subnet-mask

  • Router(config-if)# exit

• Multiple loopback interfaces can be enabled on a router.

• The IPv4 address for each loopback interface must be unique and unused by any other interface.

Verify Interface Settings

• #show ip interface brief - Displays a summary for all interfaces including the IPv4 address of the interface and current operational status.

• #show ip route - Displays the contents of the IPv4 routing table stored in RAM.

• #show running-config interface interface-id - Displays the commands configured on the specified interface.

• #show interfaces - Displays interface information and packet flow count for all interfaces on the device.

• #show ip interface - Displays the IPv4 related information for all interfaces on a router.

Filter Show Command Output

• terminal length command specifies the number of lines to be displayed. A value of 0 (zero) prevents the router from pausing between screens of output.

• Filtering commands:

  • section - Shows entire section that starts with the filtering expression

  • include - Includes all output lines that match the filtering expression

  • exclude - Excludes all output lines that match the filtering expression

  • begin - Shows all the output lines from a certain point, starting with the line that matches the filtering expression

Command History Feature

• The command history feature is useful, because it temporarily stores the list of executed commands to be recalled.

• To recall commands in the history buffer, press Ctrl+P or the Up Arrow key. Repeat the key sequence to recall successively older commands.

• To return to more recent commands in the history buffer, press Ctrl+N or the Down Arrow key.

• By default, command history is enabled and the system captures the last 10 command lines in its history buffer. Use the show history to display the contents of the buffer.

• It is also practical to increase the number of command lines that the history buffer records during the current terminal session only. Use the terminal history size user EXEC command to increase or decrease the size of the buffer.

Types of IPv4 Addresses

• Network address - The address by which we refer to the network. It is the lowest address. It has a 0 for each host bit in the host portion of the address.

• Broadcast address - A special address used to send data to all hosts in the network. It’s the highest address in the network range. It is the address in which the bits in the host portion are all 1s.

• Host addresses - The addresses assigned to the end devices in the network. Values between the network address and the broadcast address are assigned to the devices in that network.

Network Prefixes

• By adding a prefix length to the network address.

• The prefix length is the number of bits in the address that gives us the network portion.

• The subnet mask consists of 32 bits, just as the address does, and uses 1s and 0s to indicate which bits of the address are network bits and which bits are hosts bits.

Ways of Communication

• Unicast - the process of sending a packet from one host to an individual host

• Broadcast - the process of sending a packet from one host to all hosts in the network

• Multicast - the process of sending a packet from one host to a selected group of hosts

Broadcast Transmission

• Directed Broadcast- sent to all hosts on a specific network.

• Limited Broadcast- used for communication that is limited to the hosts on the local network. Routers do not forward this broadcast.

Multicast Transmission

• Hosts that wish to receive multicast data are called multicast clients. Multicast clients subscribe to the multicast group.

• Each multicast group has a single IPv4 multicast address ranging from 224.0.0.0 to 239.255.255.255.

• Addresses from 224.0.0.0 to 224.0.0.255 are reserved link local addresses and are used for multicast groups on a local network.

• Globally scoped addresses are 224.0.1.0 to 238.255.255.255 and are used to multicast data across the Internet.

• Administratively scoped addresses, also called limited scope addresses.

Experimental Addresses

• :One major block of addresses reserved for special purposes is the IPv4 experimental address range 240.0.0.0 to 255.255.255.254.

• These addresses are listed as reserved for future use.

• They cannot be used in IPv4 networks. However, these addresses could be used for research or experimentation.

Host Addresses

• The address range from 0.0.0.0 to 223.255.255.255 can be used for IPv4 hosts.

• Within this range are many addresses that are already reserved for special purposes: default route, loopback.

• Those addresses can be private or public.

Private and Public Addresses

• Private addresses are used in networks that require limited or no Internet access.

  • 10.0.0.0 to 10.255.255.255 (10.0.0.0 /8)

  • 172.16.0.0 to 172.31.255.255 (172.16.0.0 /12)

  • 192.168.0.0 to 192.168.255.255 (192.168.0.0 /16)

• Public addresses are designed to be used in the hosts that are publicly accessible from the Internet.

• NAT (Network Address Translation) allows the hosts in the network to "borrow" a public address for communicating to outside networks.

Default Route

• IPv4 default route is 0.0.0.0.

• The default route is used as a "catch all" route when a more specific route is not available.

• The use of this address also reserves all addresses in the 0.0.0.0 - 0.255.255.255 (0.0.0.0 /8) address block.

Loopback

• The loopback is a special address that hosts use to direct traffic to themselves. It’s set to 127.0.0.1.

• By using the loopback address instead of the assigned IPv4 host address, two services on the same host can bypass the lower layers of the TCP/IP stack.

• You can also ping the loopback address to test the configuration of TCP/IP on the local host.

Other Reserved Addresses

• Addresses from 127.0.0.0 to 127.255.255.255 are reserved.

• Addresses from 169.254.0.0 to 169.254.255.255 (169.254.0.0 /16) are designated as link-local addresses.

• Addresses from 192.0.2.0 to 192.0.2.255 (192.0.2.0 /24) are used for teaching and learning purposes. These addresses are called TEST-NET and are used in documentation and network examples.

Classes of IP Addresses

• Class A addresses use a fixed /8 prefix and have the most significant bit of the high-order octet equal to zero i.e from 0.0.0.0 /8 to 127.0.0.0 /8.

• Class B IP addresses use a /16 prefix and have the most significant two bits of the high-order octet are 10 i.e from 128.0.0.0 /16 to 191.255.0.0 /16.

• Class C addresses use a /24 prefix and have 110 on the three most significant bits of the high-order octet i.e from from 192.0.0.0 /16 to 223.255.255.0 /16.

• Classless Addressing: address blocks appropriate to the number of hosts are assigned to companies or organizations without regard to the unicast class.

Assigning Addresses within a Network

• End devices for users

• Servers and peripherals

• Hosts that are accessible from the Internet

• Intermediary devices

• Each of these different device types should be allocated to a logical block of addresses within the address range of the network.

Addresses for User Devices

• End devices are PCs, IP phones, printers, PDAs, …etc.

• IP addresses can be assigned either statically or dynamically.

• With a static assignment, the network administrator must manually configure the network information for a host: host IP address, subnet mask, and default gateway.

• Static addresses are useful for printers, servers, and other networking devices that need to be accessible to clients on the network.

• End user devices often have addresses dynamically assigned using DHCP.

Use of Public Addresses

• Accessible by the Internet, each of these must have a public space address associated with it.

• Router or firewall at the perimeter of the network must be configured to translate the internal address of the server into a public address.

• The use of public addresses is regulated and the company or organization must have a block of addresses allocated to it.

• Internet Assigned Numbers Authority (IANA) is the master holder of the IP addresses.

Addresses for Intermediary Devices

• Devices such as hubs, switches, and wireless access points do not require IPv4 addresses to operate as intermediary devices.

• Routers and firewall devices have an IPv4 address assigned to each interface.

• Each interface is in a different network and serves as the gateway for the hosts in that network.

ICMP Protocol

• ICMP is the messaging protocol for the TCP/IP suite.

• ICMP provides control and error messages and is used by the ping and traceroute utilities.

• ICMP messages that may be sent include:

  • Host confirmation (ping utility)

  • Unreachable Destination or Service

  • Time exceeded

  • Route redirection

  • Source quench

Ping Tool

• Ping is a utility for testing IP connectivity between hosts.

• Ping sends out requests for responses from a specified host address.

• Ping uses an ICMP Echo Request datagram.

• If the host at the specified address receives the Echo request, it responds with an ICMP Echo Reply datagram.

• Pinging the Local Loopback address (127.0.0.1) is used to test the internal configuration of IP on the local host.

Traceroute Tool

• Traceroute (tracert) is a utility that allows us to observe the path between these hosts.

• The trace generates a list of hops that were successfully reached along the path.

• Using traceroute provides round trip time (RTT) for each hop along the path and indicates if a hop fails to respond. The round trip time (RTT) is the time a packet takes to reach the remote host and for the response from the host to return. An asterisk (*) is used to indicate a lost packet.

Anding Operation

• The IPv4 host address is logically ANDed with its subnet mask to determine the network address to which the host is associated.

• Routers use ANDing to determine an acceptable route for an incoming packet.

• A host extracts its network address by ANDing its address with its subnet mask.

• If this network address of the destination host matches the network address of the local host, the packet is sent directly to the destination host. If the two network addresses do not match, the packet is sent to the gateway.

Role of an ISP

• To get access to Internet, we connect our data network to the Internet using an Internet Service Provider (ISP).

• Most companies obtain their IPv4 address blocks from an ISP.

• An ISP generally supplies a small number of usable IPv4 addresses to their customers as a part of their services.

• The ISP rents these addresses to the organization.

• If the Internet connectivity was moved to another ISP, the new ISP will provide addresses from the address blocks that have been provided to them, and the previous ISP returns the loaned blocks to their allocation to be loaned to another customer.

Subnetting

• Subnetting allows for creating multiple logical networks from a single address block.

• Since we use a router to connect these networks together, each interface on a router must have a unique network ID.

• Every node on that link is on the same network.

• We create the subnets by using one or more of the host bits as network bits.

• This is done by extending the mask to borrow some of the bits from the host portion of the address to create additional network bits.

• The more host bits used, the more subnets that can be defined.

VLSM

• When identifying the total number of hosts using traditional subnetting, we allocate the same number of addresses for each subnet.

• If all the subnets have the same requirements for the number hosts, these fixed size address blocks would be efficient. However, most often that is not the case.

• To remedy to this situation, subnetting a subnet can be done. This is called VLSM.

IPv4 Drawbacks

• IP address depletion - IPv4 has a limited number of unique public IP addresses available.

• Internet routing table expansion - As the number of servers connected to the Internet increases, so too does the number of network routes.

• Lack of end-to-end connectivity - Network Address Translation (NAT) provides a way for multiple devices to share a single public IP address.

IPv6 Benefits

• Increased address space - IPv6 addresses are based on 128-bit hierarchical addressing. This dramatically increases the number of available IP addresses.

• Improved packet handling - The IPv6 header has been simplified with fewer fields.

• Eliminates the need for NAT - Customer sites, from the largest enterprises to single households, can get a public IPv6 network address so NAT is no longer needed.

• Integrated security - IPv6 natively supports authentication and privacy

IPv6 Address

• IPv6 addresses are 128 bits in length and written as a string of hexadecimal values, meaning 32 hexadecimal values.

• IPv6 addresses are not case sensitive and can be written in either lowercase or uppercase.

• The preferred format for writing an IPv6 address is x:x:x:x:x:x:x:x, with each “x” consisting of four hexadecimal values.

• The term octet. In IPv6, a hextet is the unofficial term used to refer to a segment of 16 bits or four hexadecimal values.

Rules to Write an IPv6 Address

• Any leading 0s (zeros) in any 16-bit section or hextet can be omitted.

• A double colon (::) can replace any single, contiguous string of one or more 16-bit segments (hextets) consisting of all 0s.

• The double colon (::) can only be used once within an address

• Incorrect address: 2001:0DB8::ABCD::1234

IPv6 Address Types

• Unicast - An IPv6 unicast address uniquely identifies an interface on an IPv6-enabled device.

• Multicast - An IPv6 multicast address is used to send a single IPv6 packet to multiple destinations.

• Anycast - An IPv6 anycast address is any IPv6 unicast address that can be assigned to multiple devices. A packet sent to an anycast address is routed to the nearest device having that address.

• Unlike IPv4, IPv6 does not have a broadcast address.

IPv6 Unicast Addresses

• Global unicast: is similar to a public IPv4 address. These are globally unique, Internet routable addresses. Global unicast addresses can be configured statically or assigned dynamically.

• Link-local: used to communicate with other devices on the same local link. With IPv6, the term link refers to a subnet. Link-local addresses are confined to a single link. Their uniqueness must only be confirmed on that link because they are not routable beyond the link.

• Loopback: Similar to an IPv4 loopback address, you can ping an IPv6 loopback address to test the configuration of TCP/IP on the local host. The IPv6 loopback address is all-0s except for the last bit, represented as ::1/128 or just ::1 in the compressed format.

• Unspecified address: is an all-0s address represented in the compressed format as ::/128 or just ::

• Unique local: IPv6 unique local addresses have some similarity to private addresses for IPv4. Unique local addresses are used for local addressing within a site or between a limited number of sites. These addresses should not be routable in the global IPv6.

• IPv4 embedded: These addresses are used to help transition from IPv4 to IPv6.

IPv6 Global Unicast Addresses

• Currently, only global unicast addresses with the first three bits of 001 or 2000::/3 are being assigned.

• Note: The 2001:0DB8::/32 address has been reserved for documentation purposes, including use in examples.

• A global unicast address has three parts:

  • Global routing prefix

  • Subnet ID

  • Interface ID

IPv6 Packet

• Version: for IPv6, this field is always set to 0110.

• Traffic Class - This 8-bit field is equivalent to the IPv4 Differentiated Services (DS) field. It contains a 6-bit Differentiated Services Code Point (DSCP) value used to classify packets and a 2-bit Explicit Congestion Notification (ECN) used for traffic congestion control.

• Flow Label - This 20-bit field provides a special service for real-time applications. It can be used to inform routers and switches to maintain the same path for the packet flow so that packets are not reordered.

• Payload Length - equivalent to the Total Length field in the IPv4 header.

• Next Header - equivalent to the IPv4 Protocol field. It indicates the data payload type that the packet is carrying.

• Hop Limit: replaces the IPv4 TTL field.

• An IPv6 packet may also contain extension headers (EH), which provide optional network layer information.

IPv6 Prefix Length

• IPv6 uses the prefix length to represent the prefix portion of the address.

• The prefix length is used to indicate the network portion of an IPv6 address using the IPv6 address/prefix length.

• The prefix length can range from 0 to 128.

• A typical IPv6 prefix length for LANs and most other types of networks is /64.

IPv4 and IPv6 Coexistence

• IETF has created various protocols and tools to help network administrators migrate their networks to IPv6.

• The migration techniques can be divided into three categories:

  • Dual Stack – allows IPv4 and IPv6 to coexist on the same network. Dual stack devices run both IPv4 and IPv6 protocol stacks simultaneously.

  • Tunneling – tunneling is a method of transporting an IPv6 packet over an IPv4 network. The IPv6 packet is encapsulated inside an IPv4 packet, similar to other types of data.

  • Translation – Network Address Translation 64 (NAT64) allows IPv6-enabled devices to communicate with IPv4-enabled devices using a translation technique similar to NAT for IPv4. An IPv6 packet is translated to an IPv4 packet, and vice versa.

Static Configuration of a Global Unicast Address

• The interface command to configure an IPv6 global unicast address on an interface is ipv6 address ipv6-address/prefix-length.

Host Configuration

• Manually configuring the IPv6 address on a host is similar to configuring an IPv4 address.

• There are two ways in which a device can obtain an IPv6 global unicast address automatically:

  • Stateless Address Auto Configuration (SLAAC)

  • DHCPv6

SLAAC

• It is a method that allows a device to obtain its prefix, prefix length, and default gateway address information from an IPv6 router without the use of a DHCPv6 server.

• Using SLAAC, devices rely on the local router’s ICMPv6 Router Advertisement (RA) messages to obtain the necessary information.

Enabling IPv6 Router

• An IPv6 router is a router that:

  • Forwards IPv6 packets between networks

  • Can be configured with static IPv6 routes or a dynamic IPv6 routing protocol

  • Sends ICMPv6 RA messages

• IPv6 routing is not enabled by default. To enable a router as an IPv6 router, the ipv6 unicast-routing global configuration command must be used.

ICMPv6 RA message contains one of the following three options:

• Option 1 - SLAAC Only – The device should use the prefix, prefix-length, and default gateway address information contained in the RA message.

• Option 2 – SLAAC and DHCPv6 – The device should use the prefix, prefix- length, and default gateway address information in the RA message.

• Option 3 – DHCPv6 only – The device should not use the information in this RA message for its addressing information

Router Solicitation (RS) message

• An IPv6 device on the network does not have to wait for these periodic RA messages. A device can send a Router Solicitation (RS) message to the router, using the IPv6 all-routers multicast group address.

• When an IPv6 router receives an RS message it will immediately respond with a RA.

EUI-64 Process vs. Random Generation of Interface-ID

• If option 1 (SLAAC only) or option 2 (SLAAC with DHCPv6) is used, the client does not obtain the actual Interface ID portion of the address. The client device must determine its own 64-bit Interface ID by using one of the two methods::

  • EUI-64 process

  • Random generation

EUI-64 Process

• IEEE defined the Extended Unique Identifier (EUI) or modified EUI-64 process.

• This process uses a client’s 48-bit Ethernet MAC address, and inserts another 16 bits in the middle of the 48-bit MAC address to create a 64-bit Interface ID.

Random Generation of Interface ID

• Depending upon the operating system, a device may use a randomly generated Interface ID instead of using the MAC address and the EUI-64 process.

IPv6 Link-Local Unicast Address

• If a link-local address is not configured manually on an interface, the device will automatically create its own without communicating with a DHCP server.

• IPv6-enabled hosts create an IPv6 link-local address even if the device has not been assigned a global unicast IPv6 address.

Dynamic Assignment of a Link Local Address

• The link

/