
Configuring IP, ARP, RARP, RIP, and OSPF Services
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Addresses and Variable-Length Masks
A destination in a topology is expressed as an IP address and a variable-length
mask. Together, the address and the mask indicate the range of destinations to
which the route applies.
The ability to specify a range of networks allows the protocol to send one
summary advertisement that represents multiple destinations. For example, a
summary advertisement for the destination 128.185.0.0 with a mask of 0xffff0000
describes a single route to destinations 128.185.0.0 through 128.185.255.255.
OSPF and BGP support variable-length masks. However, RIP advertises only the
network address.
Address Resolution Protocol (ARP)
The IP router needs both a physical address and an IP address to transmit a
datagram. If the router knows only the network host’s IP address, the Address
Resolution Protocol (ARP) enables the router to determine the network host’s
physical address by binding a 32-bit IP address to a 48-bit media access control
(MAC) address. A router can use ARP across a single network only, and the
network hardware must support physical broadcasts.
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