TCP/IP Configuration and Management Manual

NonStop TCP/IP Processes and Protocols
TCP/IP Configuration and Management Manual—427132-004
B-24
Address Resolution Protocol (ARP)
through 2-172). Also, refer to the book TCP/IP Illustrated by W. Richard Stevens,
Prentice Hall, 1994.
Address Resolution Protocol (ARP)
The Address Resolution Protocol (ARP) dynamically binds a high-level Internet
Address to a low-level physical hardware address. Two machines on a single physical
network must know each other’s physical address to communicate. The ARP’s
address-resolution capability extends across only one physical network and is limited
to networks that support hardware broadcast.
Ethernet (as originally defined by DEC, Intel, and Xerox (DIX)) allows many protocols
to coexist on the same cable. Each protocol uses Ethernet packets (frames) that have
their own type specified in the type field in the Ethernet packet header. The Ethernet
specification requires the 48-bit physical (hardware) addresses that are assigned in
blocks to hardware manufacturers by the Xerox Corporation.
The Internet Protocol (IP) specification requires the 32-bit Internet (IP) addresses that
are assigned by the DDN Network Information Center (NIC) operated by SRI
International in Menlo Park, California.
To use the IP over an Ethernet network (or any other network that uses a different size
address), the two types of addresses must be resolved. The NonStop TCP/IP software
uses the Address Resolution Protocol (ARP, RFC 826) to convert 32-bit Internet (IP)
addresses into 48-bit Ethernet addresses. The ARP is a low-level protocol that
effectively hides physical addresses and allows the IP to employ 32-bit Internet
addresses. Figure B-14
shows ARP’s relationship to the Ethernet and Internet
protocols.
The caller (host A) broadcasts a special packet addressed to the receiver’s Internet
address (host B). All nodes receive the packet; however, only host B replies, and its
reply includes its physical address. When host A receives this reply, it dynamically
binds the Internet-to-physical address and caches it for subsequent use.
Figure B-14. Address Resolution Protocol
033VST .VSD
ARP Translation
Ethernet
48 Bit
Address
Internet
32 Bit
Address