1. 1 4 ARP : Address Resolution Protocol

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

3. 3 4 ARP Packet Format Frame type Ethernet dest addr Ethernet srce addr 662 Ethernet header hard type prot type hard size Sender Eth addr prot size operat Sender IP addr tartget Eth addr target IP addr 211264642 28 byte ARP request/reply Hard type - type of hardware address (cfr mac, hdlc...) Protcol Type - 0x0800 for IP Operation ARP req (1) - ARP repl (2) - RARP req (3) - RARP repl (4)

4. 4 4 Resolving a Local IP Address IP Address = 131.107.7.28 Hardware Address = 08004... IP Address = 131.107.7.29 Hardware Address = 08007... 44 ARP Broadcast ARP Broadcast 22 ARP Cache 131.107.7.7 08004... 11 ping 131.107.7.29 ARP Cache 131.107.7.7 08004... 131.107.7.28 08004... 131.107.7.7 08004... 131.107.7.28 08004... 33

5. 5 4 Resolving a Remote IP Address A Router B IP Address = 131.107.3.24 Hardware Address = 08004... IP Address = 131.107.7.29 Hardware Address = 08009... ARP Cache 131.107.3.7 08004... 11 131.107.3.1 08005... 131.107.7.1 08006... ARP for Router B ARP for Router B 77 ARP Broadcast for Router A ARP Broadcast for Router A22 ping 131.107.7.29 ARP Cache 131.107.7.7 08009... 131.107.7.1 08006... 131.107.7.7 08009... 131.107.7.1 08006... 66 33 Hardware Address of ROUTER ping 131.107.7.29 44 55 44

6. 6 4 The ARP Cache IP AddressHardware Address 131.107.255.255 =FFFFFFFFFFFF 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 22 11

7. 7 4 Viewing and Modifying the ARP Cache To Display All Entries arp -g or arp -a To Delete an Entry arp -d 131.107.3.27 To Add a Permanent Entry arp -s 131.107.3.24 02-60-8c-6c-28-93

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

9. 9 4 Exercise: Viewing and Modifying the ARP Cache

10. 10 4 Proxy ARP A Router B ARP Broadcast for 131.107.3.29 ARP Broadcast for 131.107.3.29 IP Address = 131.107.3.24 Hardware Address = 08004... IP Address = 131.107.3.29 ARP Cache 131.107.3.7 08004... 11 131.107.3.1 08005... 22 ping 131.107.3.29 ARP Cache 33 44 Hardware Address = 08005...