ARP - Address Resolution Protocol
Translates IP to MAC.
ARP broadcasts a request packet to all the machines on the LAN and asks if any of the machines are using that particular IP address. When a machine recognizes the IP address as its own, it sends a reply so ARP can update the cache for future reference and proceed with the communication.
- ARP is encapsulated in Ethernet frame:
- SRC MAC: own
- DST MAC: FF:FF:FF:FF:FF:FF
- EtherType:
0x0806- next header is ARP - ARP Header:
- Ethertype: 0x0800 - next header is IPv4 but there is no IP header
- OpCode: 1 - request, 2 - reply
- SRC MAC: own
- DST MAC: MAC we are looking for
- SRC IP: our IP
- DST IP: 0.0.0.0
Types of messages:
Request (who-has)
We use it to find MAC address for known IP address:
HW_SRC - myself
HW_DST - FF:FF:FF:FF:FF:FF - b-cast MAC
operation code - 1
Ethernet II
Destination: Broadcast (ff:ff:ff:ff:ff:ff)
Address: Broadcast (ff:ff:ff:ff:ff:ff) <<< b-cast
.... ..1. .... .... .... .... = LG bit: Locally administered address (this is NOT the factory default)
.... ...1 .... .... .... .... = IG bit: Group address (multicast/broadcast)
Source: XiaomiMobile_0c:c7:f0 (28:d1:27:0c:c7:f0)
Address: XiaomiMobile_0c:c7:f0 (28:d1:27:0c:c7:f0)
.... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
.... ...0 .... .... .... .... = IG bit: Individual address (unicast)
Type: ARP (0x0806)
Address Resolution Protocol (request)
Hardware type: Ethernet (1)
Protocol type: IPv4 (0x0800)
Hardware size: 6
Protocol size: 4
Opcode: request (1)
Sender MAC address: XiaomiMobile_0c:c7:f0 (28:d1:27:0c:c7:f0)
Sender IP address: 10.13.13.101
Target MAC address: 00:00:00_00:00:00 (00:00:00:00:00:00) <<< We are looking for this address
Target IP address: 10.13.13.122 <<< We are asking who has this IP
Reply ()
It is reply to the ARP request. If we are the owner of the IP, we should reply with our MAC address:
HW_SRC - myself
HW_DST - HW_MAC who asked
operation code - 2
Ethernet II
Destination: XiaomiMobile_0c:c7:f0 (28:d1:27:0c:c7:f0)
Address: XiaomiMobile_0c:c7:f0 (28:d1:27:0c:c7:f0) <<< u-cast
.... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
.... ...0 .... .... .... .... = IG bit: Individual address (unicast)
Source: Apple_b7:a6:ec (6c:7e:67:b7:a6:ec)
Address: Apple_b7:a6:ec (6c:7e:67:b7:a6:ec)
.... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
.... ...0 .... .... .... .... = IG bit: Individual address (unicast)
Type: ARP (0x0806)
Address Resolution Protocol (reply)
Hardware type: Ethernet (1)
Protocol type: IPv4 (0x0800)
Hardware size: 6
Protocol size: 4
Opcode: reply (2)
Sender MAC address: Apple_b7:a6:ec (6c:7e:67:b7:a6:ec) <<< We got the MAC
Sender IP address: 10.13.13.122
Target MAC address: XiaomiMobile_0c:c7:f0 (28:d1:27:0c:c7:f0)
Target IP address: 10.13.13.101
Gratuitous ARP
If we want to announce our MAC-IP binding. Usualy when the interface goes UP. It is like reply but to b-cast:
HW_SRC - myself
HW_DST - FF:FF:FF:FF:FF:FF or 00:00:00:00:00:00 - b-cast MAC
operation code - 2
Is gratuitous - True
Ethernet II
Destination: Broadcast (ff:ff:ff:ff:ff:ff)
Address: Broadcast (ff:ff:ff:ff:ff:ff) <<< b-cast
.... ..1. .... .... .... .... = LG bit: Locally administered address (this is NOT the factory default)
.... ...1 .... .... .... .... = IG bit: Group address (multicast/broadcast)
Source: Cisco_e1:a0:68 (2c:54:2d:e1:a0:68)
Address: Cisco_e1:a0:68 (2c:54:2d:e1:a0:68)
.... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
.... ...0 .... .... .... .... = IG bit: Individual address (unicast)
Type: 802.1Q Virtual LAN (0x8100)
802.1Q Virtual LAN
000. .... .... .... = Priority: Best Effort (default) (0)
...0 .... .... .... = DEI: Ineligible
.... 0000 1100 1000 = ID: 200
Type: ARP (0x0806)
Address Resolution Protocol (reply/gratuitous ARP)
Hardware type: Ethernet (1)
Protocol type: IPv4 (0x0800)
Hardware size: 6
Protocol size: 4
Opcode: reply (2)
Is gratuitous: True
Sender MAC address: Cisco_e1:a0:68 (2c:54:2d:e1:a0:68)
Sender IP address: 10.13.13.1 <<< we anounce our IP
Target MAC address: Cisco_e1:a0:68 (2c:54:2d:e1:a0:68)
Target IP address: 10.13.13.1 <<< we anounce our IP
Reverse ARP
RFC 3927, which is based on Gratuitous ARP, specifies 00:00:00:00:00:00 for the target MAC.
The client asks/requests IP from the router:
HW_SRC - myself
HW_DST - 00:00:00:00:00:00 or FF:FF:FF:FF:FF:FF - b-cast for G-ARP MAC
operation code - 3 request, 4 reply
In ARP we ask for IP of target MAC address
Ethernet II
Destination: Broadcast (ff:ff:ff:ff:ff:ff)
Address: Broadcast (ff:ff:ff:ff:ff:ff) <<< b-cast
.... ..1. .... .... .... .... = LG bit: Locally administered address (this is NOT the factory default)
.... ...1 .... .... .... .... = IG bit: Group address (multicast/broadcast)
Source: Apple_b7:a6:ec (6c:7e:67:b7:a6:ec)
Address: Apple_b7:a6:ec (6c:7e:67:b7:a6:ec)
.... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
.... ...0 .... .... .... .... = IG bit: Individual address (unicast)
Type: ARP (0x0806)
Address Resolution Protocol (reverse request)
Hardware type: Ethernet (1)
Protocol type: IPv4 (0x0800)
Hardware size: 6
Protocol size: 4
Opcode: reverse request (3)
Sender MAC address: Apple_b7:a6:ec (6c:7e:67:b7:a6:ec)
Sender IP address: 0.0.0.0
Target MAC address: Apple_b7:a6:ec (6c:7e:67:b7:a6:ec)
Target IP address: 0.0.0.0
Inverse ARP
It is used in ATM/FrameRelay networks. We use it to find IP for a given MAC address.
HW_SRC - myself
HW_DST - no idea
operation code - 8 request, 9 reply
In ARP we ask for IP of target MAC address