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FIG 6.1 Hosts Broadcasts for another Hosts MAC address

Proxy ARP

If the broadcast reaches the router it will not pass it. Routers do not forward broadcasts but if they do know how to find the host they will send their own MAC address to the host. This process is called proxy arp and it allows the host to send off the data thinking it is going straight to the remote host. The router takes care of the rest.


FIG 6.2 Router Uses Proxy ARP to Allow the Hosts to Connect

Reverse Address Resolution Protocol (RARP)

Reverse ARP maps a known MAC address to an IP address. Hosts such as diskless workstations (also known as thin clients) know their MAC address when they boot. They use RARP to discover their IP address from a server on the network.

Simple Network Management Protocol (SNMP)

SNMP is used for network management services. An SNMP management system allows network devices to send messages called traps to a management station. This informs the network administrator of any faults on the network such as faulty interfaces, high cpu utilisation on servers etc.

Domain Name System (DNS)

DNS resolves hostnames to IP addresses. This allows up to type www.cisco.com in a web browser rather than 198.133.219.25. Every ISP will have their own DNS servers which list the most common IP addresses, if the address of the server you want is not there then the ISP will pass the request onto a DNS root server.

You can configure a host name to an IP address mapping with the ‘ip host {name} {ip address}’ command:

RouterA(config)#ip host RouterB 192.168.1.2

you can then ping the hostname:

RouterA#ping routerb

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.1.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 32/32/32 ms
RouterA#

Routers automatically try to resolve an entry to a hostname if it is not an ios command. If it does not know who the hostname is, it tries to translate it:

RouterA#
RouterA#tggt
Translating "tggt"...domain server (255.255.255.255)
(255.255.255.255)
Translating "tggt"...domain server (255.255.255.255)
% Unknown command or computer name, or unable to find computer address
RouterA#

This can be very annoying because you have to wait for several seconds while the router tries to resolve the hostname. You can turn this feature off:

RouterA#conf t
Enter configuration commands, one per line. End with CNTL/Z.
RouterA(config)#no ip domain-lookup
RouterA(config)#
RouterA(config)#tggt
                 ^
% Invalid input detected at '^' marker.

RouterA(config)#