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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
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.
Network1 Network2 Network3
Table look-up
◦ 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
IP address Physical address (Hardware address)
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)
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
Is based on message exchange
Is simple. Optimizations:
◦ Caching◦ A machine broadcasts its
own IP-Ethernet binding At boot time With address resolution
request ARP message is
placed into the payload field of LAN(DLL) frame.
Hardware addr. type Protocol addr. type
Hardware addr. length
Protocol addr. length
Operation
Source hardware addr. (first 4 bytes)
Source hardware addr. (last 2 bytes)
Source protocol addr. (first 2 bytes)
Source protocol addr. (last 2 bytes)
Dest. Hardware addr. (first 2 bytes)
Dest. Hardware addr. (last 4 bytes)
Dest. Protocol addr.
ARP message
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)
ARP request message (broadcast)
ARP response message (unicast) 0x1 (Ethernet) 0x0800 (IP)
0x100 0x100000000
2 (Response)
0x0
0xE6 192.31
63.8 0x0
0xE1
192.31.65.7
0x1 (Ethernet) 0x0800 (IP)
0x100 0x100000000
1 (Request)
0x0
0xE1 192.31
65.7 0x0
0x0
192.31.63.8
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)