DK

Comprehensive Network Fundamentals Notes

Network Devices and Addressing Fundamentals

  • Audience focus: understanding network devices (hubs, switches, routers, APs), IP and MAC addressing, and core concepts used in Cisco Packet Tracer demonstrations and real networks.
  • Goals: explain roles, limitations, and interactions of devices; distinguish Collision Domain vs Broadcast Domain; explain IP vs MAC addressing; outline DHCP operations (DORA) and routing concepts; cover address classes (A–E), default gateway, and basic subnetting.

Collision Domain and Broadcast Domain

  • Collision Domain: a network segment where multiple devices contend for the same shared bandwidth; collisions can occur when two devices transmit simultaneously.
  • Hub effect: a hub forwards all frames to all ports, so all devices on the hub share a single Collision Domain and a single Broadcast Domain.
  • Switch effect: a switch forwards frames based on MAC addresses using its MAC address table; it minimizes collisions by providing a separate Collision Domain per switch port. However, a single broadcast domain is maintained across the switch unless VLANs are configured.
  • Router effect: a router breaks both Collision Domains and Broadcast Domains by routing between networks; it enables inter-network communication.
  • Access Point (AP) within a LAN: extends the LAN wirelessly; typically participates in the same Broadcast Domain as the wired side; multiple APs in a network do not by themselves create separate Broadcast Domains unless VLANs are used.
  • Practical takeaway: collisions are reduced with switches, but broadcasts still propagate within a network segment unless a router or VLAN segmentation is used.

IP Addressing vs MAC Addressing

  • IP Address (IPv4): logical address used to identify a device on an IP network; 32-bit address normally written as four octets in dotted decimal notation; composed of Network Identifier (NID) + Host Identifier (HID) parts.
  • MAC Address (Physical Address): 48-bit hardware address burned into network interface hardware; universally unique; formatted as six octets (e.g., 00:1A:2B:3C:4D:5E); not routable across networks.
  • Key differences:
    • IP is logical and routable; MAC is physical and not routable.
    • IP can be changed by configuration; MAC is fixed for a NIC (though it can be spoofed or changed in software for some stacks).
    • IP addressing is hierarchical (NID/HID concepts); MAC addressing relies on OUI (Organizationally Unique Identifier) plus device-specific bits.
  • MAC address structure: 48 bits total; first 24 bits constitute the OUI (vendor identifier); last 24 bits identify the NIC.
  • IP address usability examples:
    • Class C example: an IP address with a /24 network (default mask 255.255.255.0) has 8 host bits; usable hosts per subnet = 2^8 - 2 = 254.
    • The two addresses in every subnet reserved for network and broadcast addresses; hence 254 usable hosts in a typical Class C subnet.
  • Address assignment limits (Class C context): with a /24 network, the range is 0-255 in the host portion, but 0 (network) and 255 (broadcast) are not usable for hosts, leaving 254 usable host addresses.

IP Addressing and Address Classes (A–E)

  • IP addresses are divided into classes with default subnets:
    • Class A: NID uses 8 bits, HID uses 24 bits; Default mask 255.0.0.0 (/8); Range for usable networks: 1.0.0.0 to 126.0.0.0; 126 networks; 127.0.0.0 reserved (loopback). Example: 10.0.0.0/8.
    • Class B: NID uses 16 bits, HID uses 16 bits; Default mask 255.255.0.0 (/16); Range 128.0.0.0 to 191.255.0.0; 16,384 networks; 65,534 hosts per network.
    • Class C: NID uses 24 bits, HID uses 8 bits; Default mask 255.255.255.0 (/24); Range 192.0.0.0 to 223.255.255.0; 2,097,152 networks; 254 hosts per network.
    • Class D: Range 224.0.0.0 to 239.255.255.255; Reserved for multicast; not used for regular host addressing.
    • Class E: Range 240.0.0.0 to 255.255.255.255; Experimental/reserved for future use.
  • CIDR and subnetting later refine these classful boundaries; practical networks often use CIDR to tailor networks more precisely.
  • NOTE: In IPv4, the network portion (NID) and host portion (HID) are defined by the subnet mask; default classful masks are used only for historical reference.

DHCP and DORA

  • DHCP (Dynamic Host Configuration Protocol): automates IP address assignment to hosts.
  • DORA: Discover, Offer, Request, Acknowledge – the four-step DHCP process:
    • Discover: a client broadcasts a DHCPDISCOVER message to locate a DHCP server.
    • Offer: a DHCP server responds with an IP address offer (DHCPOFFER).
    • Request: the client requests an IP address (DHCPREQUEST).
    • Acknowledge: the server acknowledges and confirms the lease (DHCPACK).
  • DHCP Relay Agent: used when clients are on different networks than the DHCP server; the relay agent forwards DHCP messages between clients and servers, typically using unicast toward the server.
  • DHCP in networks without a DHCP server: a router can run a DHCP server compatible service or a dedicated DHCP server can be added; relay agents become essential for cross-subnet DHCP communications.
  • DHCP in Cisco Packet Tracer: you can simulate a DHCP server, configure scopes, and observe address allocation; the router can act as a relay agent if needed.
  • Key terms: Access Point (AP), AUI Port, Broadcast Domain, CLI, HID, Hub, Inter-networking, iOS, LAN, MAC Address, MAC Table, Modular Router, Multicasting, NID, Octet, Packet Tracer, Privilege Mode, Relay Agent, Router, Serial Port, Static IP, Subnet Mask, Switch, Unicasting, User Mode, WAN, WLC.

Routing Concepts and Inter-networking

  • Router purpose: inter-networking device; connects different networks and forwards packets between them; maintains routing tables and policies.
  • Inter-networking vs Intra-networking:
    • Intra-networking: within the same network (LAN) where devices communicate directly via switches/hubs.
    • Inter-networking: communication across different networks, enabled by routers.
  • Default Gateway: the IP address of the router interface on a local network; used by hosts to reach destinations outside their own LAN; essential for outbound traffic to other networks.
  • Modular Router vs Fixed Router:
    • Modular Router: supports adding/removing expansion modules (ports or features) to scale capacity; greater flexibility for port density and functionality.
    • Fixed Router: has a fixed number of ports; simpler, potentially cheaper, but less scalable.
  • WAN and Serial Port:
    • WAN: Wide Area Network; networks spread over large geographic areas.
    • Serial Port: used for WAN connectivity between routers; common in enterprise/mid-size networks.
  • Inter-networking devices in practice: routers interconnect different LANs, VLANs, or subnets; switches connect devices within a single LAN; APs extend reach wirelessly; hubs are rarely used in modern networks but illustrate shared collision domains.

Subnetting, Subnet Masks, and Network Design Basics

  • Subnet Mask: 32-bit number used with an IP address to determine the network portion; written in dotted decimal; example: 255.255.255.0 (/24).
  • Binary/subnet math (illustrative): the mask determines how many host bits remain for addressing; for a /24 mask, there are 8 host bits, yielding 256 total addresses per subnet, with 254 usable hosts.
  • Default Gateway: the router interface on the LAN side; required for hosts to reach outside networks.
  • Network Design considerations: balancing growth, security, and manageability; segmentation with routers/VLANs reduces Broadcast Domains and improves performance.

Interfaces, Ports, and Device Interfaces

  • AUI Port (Attachment Unit Interface): legacy LAN connection port on routers; used for early Ethernet interfaces to connect to external media.
  • Serial Port: WAN connection between routers; used for site-to-site links in many WAN topologies.
  • Console Port: CLI access to configure and manage a router or switch; essential for initial configuration and troubleshooting.
  • Default Gateway and LAN connections: hosts on a LAN use the default gateway to reach off-LAN destinations; routers break networks apart and manage inter-network routing.
  • Broadcast Domain management: routing devices (routers) separate Broadcast Domains; switches do not unless VLANs are configured.

Wireless Networking Basics

  • Access Point (AP): wireless network device that provides Wi-Fi connectivity; connects to the wired LAN, typically via a switch or WLC; enables wireless clients to participate in the same IP network.
  • Wireless LAN Controller (WLC): central control plane for multiple APs in an enterprise wireless deployment; manages security, RF planning, and traffic forwarding decisions.

Key Networking Concepts and Practical Implications

  • Ethernet speeds: Fast Ethernet operates at 100 Mbps.
  • Filtration vs Forwarding vs Flooding on switches:
    • Filtration: dropping frames based on MAC address filtering rules; part of switch behavior to enforce security or restrictions.
    • Forwarding: delivering frames to the correct destination port based on the MAC table.
    • Flooding: when the destination MAC is unknown, the switch floods the frame to all ports in the same collision domain to reach the destination.
  • MAC Table: maintained by switches to map MAC addresses to specific switch ports; enables efficient forwarding and reduced unnecessary traffic.
  • Graphical vs CLI interfaces: GUI is common for visual management; CLI provides command-line control for advanced configurations; Privilege Mode grants higher permissions for configuration tasks.
  • DHCP and network deployment realities: in real networks, many devices rely on DHCP to obtain an IP; Relay Agents extend DHCP functionality across subnets; Always ensure proper scope configuration and server reachability.
  • Practical research and exam focus: be able to explain what each device does, how domains are partitioned, the difference between IP and MAC addresses, and the purpose of DHCP operations (DORA) and routing.

Quick Q&A (2–3 sentence summaries)

  • What is a Collision Domain and how does it affect network performance?
    • A Collision Domain is a network segment where multiple devices share a single collision space. In hubs, all devices share one Collision Domain, leading to more collisions and reduced performance; switches limit collisions by providing a separate Collision Domain per port; routers further segment collision domains by routing between networks.
  • How does a hub, switch, and router differ in terms of connectivity and domain separation?
    • A hub repeats frames to all ports, creating a single Collision Domain and a single Broadcast Domain. A switch uses MAC addressing to forward frames and creates multiple Collision Domains (one per port) while maintaining a single Broadcast Domain (unless VLANs are used). A router connects different networks and breaks both Collision Domains and Broadcast Domains, enabling inter-network communication.
  • Why is a switch called a ‘multieport device’ and what is its Collision Domain behavior?
    • A switch has multiple ports (multieport) and, importantly, each port represents its own Collision Domain. Practically, a switch reduces collisions compared to a hub, since devices on different ports do not compete for the same collision space.
  • What is the main purpose of a router in a network, and how does it operate across networks?
    • A router connects different networks and routes packets between them using routing tables and policies; it forwards traffic from one IP network to another and isolates Broadcast Domains across networks.
  • What does the Default Gateway mean, and why is it necessary beyond the LAN?
    • The Default Gateway is the router’s IP address on the local network used to reach destinations outside the LAN; it is necessary to forward traffic to other networks and reach remote hosts or Internet resources.
  • What are Modular vs Fixed routers, and when would you choose one over the other?
    • Modular routers allow adding/removing modules to expand ports and features, offering scalability; fixed routers have a fixed port count and are simpler and cheaper but less scalable. Choose based on current and projected growth, port density needs, and feature requirements.
  • How do IP addresses differ from MAC addresses, and why are both needed?
    • IP addresses are logical, routable identifiers used for end-to-end communication across networks; MAC addresses are physical, hardware identifiers used for local delivery on a single network segment. Routers use IPs to route across networks, while switches use MACs to forward within a local network.
  • What is the typical usable host count in a Class C network, and why is it 254?
    • In a typical Class C network with a /24 mask, there are 256 total host IDs (2^8), but two addresses are reserved (network and broadcast), leaving 254 usable hosts: 2^8 - 2 = 254.