09 ARP
Address Resolution Protocol
Obtain MAC Address from IP address
It is ‘plug-and-play’, as nodes create their ARP tables w/o intervention from network admin
ARP Procedure¶
Let’s say node A wants to send message to node B
- Check ARP Table for B’s MAC address using IP address.
- If B found, A sends message to B. Stop
- Else, A broadcasts ARP query packet, containing B's IP address
- dest MAC address = \(FF \ FF \ FF \ FF \ FF \ FF\)
- all nodes on LAN receive ARP query
- B receives ARP packet, replies to A with its (B's) MAC address frame sent to A’s MAC address (unicast)
- A adds B’s mac address to its ARP table
- Go to step 1
| ARP Message Type | Type |
|---|---|
| ARP Request | Broadcast |
| ARP Reply | Unicast |
ARP Table¶
Each IP node on LAN has ARP table, which contains
| IP Address | MAC Address | TTL |
|---|---|---|
| Time To Live Time after which address mapping will be forgotten Usually 20min |
ARP Packet¶
Destination address of the frame will be all 1s, as this will be broadcast
| Meaning | Value | Size | |
|---|---|---|---|
| Hardware Type | Type of network on which ARP is running | Ethernet -> 1 | 16 bits |
| Hardware Length | Length of physical address in bytes | Ethernet -> 6 | 8 bits |
| Protocol Length | Length of logical address in bytes | 8 bits | |
| Protocol Type | Type of IP used | IPv4 -> \(0800_H\) | 16 bits |
| Operation | Type of ARP Packet | Request -> 1 Reply -> 2 | 16 bits |
| Sender MAC address | 6 bytes for ethernet | ||
| Sender IP address | 4 bytes for IP | ||
| Target MAC address | Request (all 0s as MAC address unknown) Reply (returned MAC address) | 6 bytes for ethernet | |
| Target IP address | 4 bytes for IP |
Target IP address¶
| Sender | Receiver | Network | Value of Target IP address |
|---|---|---|---|
| Host | Host | Same | Same as destination IP address in IP packet |
| Host | Host | Different | IP address of router |
| Router | Host | Same | Same as destination IP address in IP packet |
| Router | Host | Different | IP address of router |