Address resolution ARP Protocol

• ARP Protocol belongs to network layer protocol
• When a host sends Ethernet data frames to another host on the same LAN , It's based on mac Address to determine the interface .
• ARP The agreement is to settle in a LAN in , host A Host B Of IP Address , But there is no host B Of MAC The question of address

1. Application scenarios ：

When one LAN There is a host in it A（IP: 192.168.1.1） And host B（IP: 192.168.1.2）, here ： host A Request initiation TCP Connect to host B.

1. When the host A It is necessary to send messages to the host B（IP:192.168.1.2）, host A Check out your Arp surface （arp -a You can see ）, Use it if you have it Arp Host of table B Of mac Address .
2. There is no host B Of IP Address corresponding MAC Address , This time the mainframe A Will send a ARP Request message （ radio broadcast ）
3. host B And this LAN Other hosts in received this Arp Broadcast message , host B Will reply ARP Reply message （ Include host B Of IP Address and MAC Address ） Host computer A（ unicast ）, Other hosts do not respond .
4. host A Will put the host B Of IP and MAC Address deposit ARP surface （ There is a timeout deletion mechanism ）

2. ARP Message format

•  ARP Message format

host A stay LAN Internal broadcast ARP message ,LAN The host inside B reply ARP response message .

1.   Ethernet destination address ： All for 1 On behalf of broadcast address , Send to LAN All hosts in the
2. Ethernet source address ： The sender （ host A） Of MAC Address
3. The frame type ：ARP request / Reply message ; The frame type is 0x0806
4. Hardware type ： 0001 For Ethernet
5. Protocol type ：0800 representative IPV4 agreement
6. Hardware address length ： Sending end MAC Address length .
7. Protocol address length ： Length of protocol address at the sending end , It's usually IP agreement , Corresponding IP Address length , Namely 0000 0100
8. op Field ： ARP Request for 1,ARP Answer is 2,RARP Request for 3,RARP Answer is 4.
9. Ethernet address of sending end ： The sender （ host A） Of MAC Address .
10. The sender IP Address ： The sender （ host A） Of IP Address .
11. Destination ethernet address ： Target end （ host B） Of MAC Address . whole 0
12. Purpose IP Address ： Target end （ host B） Of IP Address .

3. ARP Packet capture

• ARP Request message ： host A（192.168.1.1） Send to LAN The host inside B（192.168.1.123）

•  ARP response message