Lecture on Local Access Networks

Introduction to Local Access Networks

• Focus on link layer MAC addresses and ARP (Address Resolution Protocol).
• Discussion on different versions of the Ethernet protocol.

MAC Addressing

• Characteristics:
  • MAC addresses are Layer 2 addresses, used locally within one subnet.
  • 48-bit addresses embedded in network interface card (NIC), can sometimes be changed via software.
  • Written in pairs of hexadecimal digits (e.g., XX-XX-XX-XX-XX-XX or XX:XX:XX:XX:XX:XX).
  • First few bits identify the manufacturer (OUI - Organizationally Unique Identifier).
  • Significance is limited to local subnet; used for exact matching rather than longest prefix matching.

Address Resolution Protocol (ARP)

• Functionality:
  • Maps IP addresses to MAC addresses within the same subnet.
  • Each device maintains an ARP table with IP-MAC address pairs.
  • ARP table entries have a TTL (Time to Live), typically 20 minutes.
• ARP Process:
  • Host A sends a broadcast ARP message to find MAC address of Host B.
  • All devices on network receive it; only Host B responds with MAC address.
  • Routers do not forward ARP broadcasts, limiting them to the local subnet.
  • If Host A and B are on different subnets, Host A uses its gateway router's MAC address for communication.

Ethernet Overview

• Dominance in LAN technology:
  • Simple design using CSMA/CD protocol.
  • Supports multiple speeds (up to 400 Gbps) and media types.
• Evolution:
  • Transitioned from bus topology to hub, then to switched networks.
  • Switched networks eliminate collisions, increasing efficiency.
• Ethernet Frame Structure:
  • Includes 8-byte preamble for clock synchronization.
  • Contains source and destination MAC addresses, type field, and CRC for error checking.
  • Utilizes a broadcast and select model for frame processing.

Ethernet Standards

• Physical Media and Speeds:
  • Historical use of coaxial cables, now commonly RJ45 with twisted pairs or fiber optics.
  • Standards vary by speed and medium (e.g., 100 Mbps, 1 Gbps, 10 Gbps, 40 Gbps).

Conclusion

• Next topics include Layer 2 switches and VLANs.

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