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TOPIC: dhcp

dhcp 14 years 4 months ago #2850

  • indebluez
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hey guys...
1)whats DHCP...?

2) and why do we need a secondary address for the ethernet interface on our routers?


Re: dhcp 14 years 4 months ago #2852

DHCP stands for Dynamic Host Configuration Protocol. It is a protocol whereby a host can receive IP configuration details from a nearby DHCP server. Instead of having to set these details manually on each workstation, the DHCP server can provide a host with an IP address, default gateway, DNS server, WINS server, etc. For a humourous look at DHCP, check out . The page includes details on how to configure a Cisco Router to provide DHCP services.


Re: dhcp 14 years 4 months ago #2853

1. Greenice wins the 'First Correct Answer I've Seen This Morning' award. Congratulations, contact tfs for your free tshirt.

I'll also give the 'You have elite skills' award to the person who can properly differentiate between BOOTP and DHCP (hint: DHCP is actually an extension of BOOTP).

2. Secondary address ? Meaning ? Do you mean why do we set an IP address for the ethernet interface ? (You ask me a question and I reply with three questions) If we didn't give it an IP address then everything would be kaput. Cmon now, we can do better than this !!

Re: dhcp 14 years 4 months ago #2860

  • indebluez
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dear sahir!

i did not ask why we have ip my ccnp class lab session, we applied a primary and secondary address for theinterface...
so when we running the PROTOCOL INSPECTOR, for troubleshooting,
instead of showing one IP address it showed two addresses....thats why i was askin whats a secondary addfress...
and whats BOOTP?


Re: dhcp 14 years 4 months ago #2862

BOOTP is the bootstrap protocol, it allows a 'dumb' machine that does not have a hard-disk or any such storage media to find the address of a BOOTP server as well as its own IP address and then download a file that it will load into memory and use for booting.

Basically it downloads its brains from the BOOTP server..
RFC 951 has the full specification.

Re: dhcp 14 years 4 months ago #2873

The following scenario should illustrate the purpose of a secondary address:

Your network is using an ip addressing scheme of Subnet has had all 254 host addresses allocated, yet you now require more addresses in this subnet. What do you do? Assuming that subnet has already been allocated, you are quite limited in what you can do. Enlarging the subnet by changing the subnet mask will result in an overlap with, so that won't work. You could change the subnet mask of the whole network to give you more hosts, but that would require you to change the ip config of each existing host. The simplest solution is to add a secondary address to the interface of the subnet in question, thereby adding a second subnet with another 254 hosts. Let's say you add subnet So, you now have two subnets hanging off the one interface, and, with the router forwarding packets between the two. I hope that makes sense.

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