Computer Network and Security - Exam Notes

Address Resolution Protocol (ARP)

• Used to determine the destination MAC address of a local device given its IPv4 address.
• Functions:
  • Resolving IPv4 addresses to MAC addresses.
  • Maintaining an ARP table (IPv4 to MAC mappings).
• To send a frame, a device checks its ARP table. If no entry is found, an ARP request is sent.
• Commands to view ARP table:
  • Cisco router: show ip arp
  • Windows 10 PC: arp -a
• Vulnerabilities: ARP spoofing, mitigated by enterprise-level switches.

Network Layer

• Provides services for end devices to exchange data.
• Protocols: IPv4 and IPv6.
• Basic operations:
  • Addressing end devices
  • Encapsulation
  • Routing
  • De-encapsulation
• IP encapsulates the transport layer segment; IP addressing doesn't change from source to destination (except with NAT).

IPv4 Packet

• Header ensures packets are sent correctly and contains info for layer 3 processing.
• Header fields include version, differentiated services, header checksum, TTL, protocol, source IPv4 address, and destination IPv4 address.
• Limitations:
  • Address depletion
  • Lack of end-to-end connectivity due to NAT
  • Increased network complexity from NAT

IPv6 Packet

• Developed to overcome IPv4 limitations.
• Improvements:
  • Increased address space (128-bit addresses)
  • Simplified packet handling
  • Elimination of NAT
• Header is fixed at 40 bytes and simplified.
• Significant fields: version, traffic class, flow label, payload length, next header, hop limit, source IPv6 address, destination IPv6 address.

Host Forwarding Decision

• Packets are created at the source; each host has a routing table.
• A host can send packets to itself (loopback addresses $127.0.0.1$ for IPv4, $::1$ for IPv6), local hosts, or remote hosts.
• Source determines if destination is local or remote based on IP address and subnet mask (IPv4) or network address and prefix (IPv6).
• Remote traffic is forwarded to the default gateway.

Default Gateway (DGW)

• Must have an IP address in the same range as the LAN.
• Accepts data from the LAN and forwards traffic off-LAN.
• Can route to other networks.
• Configured statically or via DHCP (IPv4) or router solicitation (IPv6).
• Acts as a static route (last resort) in the routing table.

IP Router Routing Table

• Types of routes:
  • Directly Connected: Automatically added for active interfaces with addressing.
  • Remote: Learned manually (static route) or dynamically (routing protocol).
  • Default Route: Forwards unmatched traffic in a specific direction.

Static vs Dynamic Routing

• Static Routing: Configured manually, good for small networks.
• Dynamic Routing: Automatically discovers networks and adapts to topology changes.
• Command show ip route shows route sources (L, C, S, O, D) and types (directly connected, remote, default).

IPv4 Address Structure

• 32-bit hierarchical address with network and host portions.
• Subnet mask determines network and host portions.
• Prefix length identifies the subnet mask (e.g., /24).

Types of IPv4 Addresses

• Network Address: All host bits are 0.
• Host Addresses: Range between the first and last address.
• Broadcast Address: All host bits are 1.
• Public: Globally routed between ISPs.
• Private: Non-routable, used internally (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16).
• Loopback: $127.0.0.0 /8$, used for testing TCP/IP stack.
• Link-Local: $169.254.0.0 /16$, self-assigned when DHCP is unavailable (APIPA).

IPv4 Address Classes (Historical)

• Class A: $0.0.0.0/8$ to $127.0.0.0/8$
• Class B: $128.0.0.0 /16$ to $191.255.0.0 /16$
• Class C: $192.0.0.0 /24$ to $223.255.255.0 /24$
• Class D: 224.0.0.0 to 239.0.0.0
• Class E: 240.0.0.0 – 255.0.0.0
• Classful addressing is obsolete; classless addressing is now used.

Network Segmentation

• Broadcast Domain: Area where broadcasts are propagated; routers do not forward broadcasts.
• Subnetting: Reduces broadcast domain size, improving network performance and security.

Subnetting IPv4 Networks

• Octet Boundary: Common subnetting at /8, /16, and /24.
• Within Octet Boundary: Subnetting using masks like /25, /26, /27, etc.
• Private vs. Public: Intranet uses private addresses, DMZ uses public addresses.

VLSM (Variable Length Subnet Masking)

• Subnetting a subnet to efficiently allocate addresses.
• Start with largest subnet requirements and subnet until smallest subnet requirements are met.

IPv4 Network Address Planning

• Crucial for scalable network design.
• Consider:
  • Number of subnets needed
  • Number of hosts per subnet
  • Device types
  • Private vs. public addresses

Device Address Assignment

• End User Clients: Use DHCP.
• Servers and Peripherals: Static IP addresses.
• Internet-Accessible Servers: Public IPv4 addresses (often using NAT).
• Intermediary Devices: Addresses for management.
• Gateway: Routers and firewalls.