ARP Address Resolution Protocol 4 1 Introduction Problem

ARP : Address Resolution Protocol 4 1

Introduction - Problem n Successful Mapping of an IP Address to a Hardware Address IP uses IP-address Ethernet uses MAC-address 2 4

ARP Packet Format hard size prot size Ethernet dest addr 6 Ethernet Frame srce addr type 6 Ethernet header 2 prot type 2 operat 1 1 2 Sender Eth addr 6 Sender IP addr tartget Eth addr 4 6 target IP addr 4 28 byte ARP request/reply n Hard type - type of hardware address (cfr mac, hdlc. . . ) n Protcol Type - 0 x 0800 for IP n Operation l 3 2 hard type ARP req (1) - ARP repl (2) - RARP req (3) RARP repl (4) 4

Resolving a Local IP Address ping 131. 107. 7. 29 1 3 ARP Cache 131. 107. 7. 7 08004. . . 131. 107. 7. 28 08004. . . 2 ARP Broadcast Hardware Address = 08007. . . IP Address = 131. 107. 7. 28 Hardware Address = 08004. . . 4 4 IP Address = 131. 107. 7. 29 Hardware Address = 08007. . . 4

Resolving a Remote IP Address ping 131. 107. 7. 29 6 1 ARP Cache 131. 107. 7. 7 08009. . . 131. 107. 7. 1 08006. . . 4 ping 131. 107. 7. 29 131. 107. 3. 7 08004. . . 2 ARP Broadcast for Router A ARP Cache 131. 107. 3. 1 08005. . . A Router 3 Har re A a w d IP Address = 131. 107. 3. 24 Hardware Address = 08004. . . 5 ping 131. 107. 7. 1 4 131. 107. 08006. . . 7. 29 B 5 R TE U O f. R o s s e ddr ARP Cache 7 ARP for Router B IP Address = 131. 107. 7. 29 Hardware Address = 08009. . . 4

The ARP Cache IP Address Hardware Address 131. 107. 255 = FFFFFF 131. 107. 3. 5 = 080009654321 131. 107. 3. 24 = 080004321371 131. 107. 78. 3 = 080006723111 131. 107. 9. 4 = 080002345621 131. 107. 7. 29 = 080009654441 2 1 4 6

Viewing and Modifying the ARP Cache n To Display All Entries arp -g or arp -a n To Delete an Entry arp -d 131. 107. 3. 27 n To Add a Permanent Entry arp -s 131. 107. 3. 24 02 -60 -8 c-6 c-28 -93 4 7

Common Address Resolution Problems n n 8 Duplicate IP Addresses l Windows NT–based hosts will not initialize l Wrong host might respond to requests l Hosts could hang Invalid Subnet Masks l Search for hosts could create unnecessary broadcasts l Unnecessary broadcasts could result in a broadcast storm l Broadcasts could decrease performance l Broadcast storm could cause hosts to hang 4

Exercise: Viewing and Modifying the ARP Cache 4 9

Proxy ARP ping 131. 107. 3. 29 1 ARP Cache 131. 107. 3. 7 08004. . . 2 ARP Broadcast for 131. 107. 3. 29 131. 107. 3. 1 08005. . . ARP Cache 3 A Router B 4 Hardware Address = 08005. . . IP Address = 131. 107. 3. 24 Hardware Address = 08004. . . 10 IP Address = 131. 107. 3. 29 4