2.  Transmission on LAN’s requires knowledge of
destination hardware address (physical address, MAC
address).
 On a LAN hardware addresses are unique.
 Address resolution is translation of destination IP
address to destination hardware address

3. Network1 Network2 Network3
 Ex 1: A->B tranmission: A resolves B’s MAC address
based on B’s IP address
 Ex 2: A->D transmission: A resolves Y1’s MAC address
based on Y1’s IP address, Y1 resolves Y2’s MAC address
based on Y2’s IP address, Y2 resolves D’s MAC address
based on D’s IP address.

4.  Table look-up IP address Physical address (Hardware address)
◦ Each node keeps a separate table
◦ Network adress part of IP address need not be stored.
◦ If address mapping changes all nodes update their tables

5.  Computation
◦ Derivation from the host address part of IP address
◦ Useful in network that employ reconfigurable (dynamical) port
address.
IP address Physical address(Hardware address)

6.  Message exchange
◦ A server responds to address resolution request that it receives
◦ Any machine that knows the hardware address responds to
address resolution request broadcasts

7.  Is based on message
exchange ARP message
 Is simple. Hardware addr. type Protocol addr. type
 Optimizations: Hardware addr.
length
Protocol
addr. length
Operation
◦ Caching Source hardware addr. (first 4 bytes)
◦ A machine broadcasts its Source hardware addr. (last 2
bytes)
Source protocol addr. (first 2
bytes)
own IP-Ethernet binding Source protocol addr. (last 2 Dest. Hardware addr. (first 2
 At boot time bytes) bytes)
 With address resolution Dest. Hardware addr. (last 4 bytes)
request Dest. Protocol addr.
 ARP message is placed
into the payload field of
LAN(DLL) frame.

8. Three interconnected /24 networks: two Ethernets and an
FDDI ring.
Two possibilities for transmissions to other networks:
• 65.7 broadcasts ARP request for 63.8, to which 65.1 responds with
(63.8-E3)
2. 65.7 realizes 63.8 is on a different network and binds (63.8-E3)

9. 0x1 (Ethernet) 0x0800 (IP)
 ARP request message 0x100 0x10000000
0
1 (Request)
(broadcast) 0x0
0xE1 192.31
65.7 0x0
0x0
 ARP response message 192.31.63.8
(unicast) 0x1 (Ethernet) 0x0800 (IP)
0x100 0x10000000 2 (Response)
0
0x0
0xE6 192.31
63.8 0x0
0xE1
192.31.65.7

10.  A diskless workstation learns its IP address based on
its Ethernet address
◦ Workstation broadcasts its Ethernet address.
◦ RARP server responds by table look-up and sending back
the corresponding IP address
◦ Since the operating system image is IP address independent,
a single image kept at a remote server can be downloaded at
boot time.
 Since broadcasts are not forwarded by routers a
single RARP server is needed on each network (or
subnet)