Skip to main content

Computer Networking: MAC Addressing and ARP

Media Access Control Address

  • Identifies the physical network interface of a host
    • Hexadecimal addressing: 0-F
    • 48 bits (6 bytes)
  • In early, primitive networks, computers were often on a shared "media" and needed a way to address each other
    • On a shared medium, all computers will receive the data
    • Network interfaces are supposed to (see promiscuous mode) ignore anything not addressed to them
    • This behavior still exists to this day

 <------------------------------------------------- 6 OCTETS ---------------------------------------------------->
                                                    48 BYTES
 _________________________________________________________________________________________________________________
|                  |                  |                  |                  |                  |                  |
|      OCTET       |      OCTET       |      OCTET       |      OCTET       |      OCTET       |      OCTET       |
|     8 BITS       |     8 BITS       |     8 BITS       |     8 BITS       |     8 BITS       |     8 BITS       |
|     1 BYTE       |     1 BYTE       |     1 BYTE       |     1 BYTE       |     1 BYTE       |     1 BYTE       |
|                  |                  |                  |                  |                  |                  |
|    00 thru FF    |    00 thru FF    |    00 thru FF    |    00 thru FF    |    00 thru FF    |    00 thru FF    |
|                  |                  |                  |                  |                  |                  |
'__________________'__________________'__________________'__________________'__________________'__________________'
 |                                                      | |                                                      |
 |                                                      | |                                                      |
 |______________________________________________________| |______________________________________________________|
                             |                                                        |
                             |                                                        |
        Organizationally Unique Identifier (OUI)                         Network Interface Card (NIC)
                         3 OCTETS                                                 3 OCTETS

Broadcast Domain vs Collision Domain

  • In the diagram below, the broadcast domain is designated as the entire Local Area Network (LAN)
    • When a host sends a layer 2 broadcast -- FF:FF:FF:FF:FF:FF -- every host on the LAN receives it
  • In the diagram below, the collision domain is designated as the shared media between hosts, the cable
    • Traffic from both switches can be on the cable in duplex, and therefore has the possibility to collide
                     ____________________________
                    |                            |
                    |                            |
 -----------------> |         SWITCH 1           |
 |                  |                            |
 |                  |                            |
 |                  '____________________________'
 |                     |   |   |   |   |   |   | 
 |                     |   |   |   |   |   |   |
 |                    PC0  |  PC2  |  PC4  |   |
 |                         |       |       |   |
 |--------------->        PC1     PC3     PC5  |
 |                                             |
 |                                             |
 |  BROADCAST               LOCAL AREA         |            COLLISION
 |     DOMAIN              NETWORK (LAN)       | <--------- DOMAIN
 |                                             |
 |                                             |
 |--------------->    PC6     PC8              |
 |                     |       |               |
 |                     |  PC7  |   PC9         |
 |                     |   |   |    |          |
 |                     |   |   |    |          |
 |                   __|___|___|____|__________|_
 |                  |                            |
 |                  |                            |
 '----------------> |         SWITCH 2           |
                    |                            |
                    |                            |
                    '____________________________'

Address Resolution Protocol

  • IP addressing is the primary way we identify hosts belonging to a network
    • It's easier to remember than a MAC address
  • When preparing to transmit data, to another host on the network, the sender needs to know two things:
    • The recipient IP address
    • The recipient MAC address
  • ARP is the protocol that is used to look up a MAC address for any given IP address
    • Send a layer 2 broadcast -- FF:FF:FF:FF:FF:FF

Who has 192.168.1.10?
Tell 192.168.1.13

    • Then, 192.168.1.10 should respond to 192.168.1.13 with its MAC address.

When we abstract things with the OSI Model or the TCP/IP Model, there's one practice I dislike
We tend to say, "MAC addresses are for communicating at layer 2".
Conveys that computers can distinctly communicate with one another using strictly MAC addresses
That's just simply not a true, as a fully formed packet must be transmitted between hosts

The diagram below is used to reinforce that point. A computer doesn't communicate with just a MAC or just an IP. It's a fully assembled frame encapsulated from the application layer to the data link layer. The TCP packet is encapsulated in the IP packet, which is encapsulated in the Ethernet frame.

encapsulation-example.png

Example shows how multiple segments of networking data are nested to form a complete Ethernet frame
https://tldp.org/LDP/tlk/net/net.html

Routing and Switching Flowchart

Computer Networking Flowchart (benheater.com)

Virtual Local Area Networks

  • VLANs are a layer 2 concept
    • 802.1q is the protocol that defines VLAN
    • The 802.1q header is injected into the Ethernet frame of a packet
    • 802.1q ID can range from 1 to 4094, due to limits on the maximum size of an Ethernet frame
  • Not going to do a deep dive on VLANs, but hope to clear some things up