1 4 ARP : Address Resolution Protocol

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

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 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 Resolving a Local IP Address IP Address = Hardware Address = IP Address = Hardware Address = ARP Broadcast ARP Broadcast 22 ARP Cache ping ARP Cache

5 4 Resolving a Remote IP Address A Router B IP Address = Hardware Address = IP Address = Hardware Address = ARP Cache ARP for Router B ARP for Router B 77 ARP Broadcast for Router A ARP Broadcast for Router A22 ping ARP Cache Hardware Address of ROUTER ping

6 4 The ARP Cache IP AddressHardware Address =FFFFFFFFFFFF = = = = =

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

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 4 Exercise: Viewing and Modifying the ARP Cache

10 4 Proxy ARP A Router B ARP Broadcast for ARP Broadcast for IP Address = Hardware Address = IP Address = ARP Cache ping ARP Cache Hardware Address =