Cisco CCNA Certification Exam Tutorial: DNS And The IP Name-Server Command
By: Chris Bryant, CCIE #12933

DNS behaviors of a Cisco router are important topics for both the CCNA exam and real-world production networks, and you probably didn't know there were so many DNS details before you began studying for the exam! In this tutorial, we'll look at the ip name-server command and its proper usage.

When a command is mistyped on a Cisco router, the default behavior of the router is to attempt to resolve it via DNS. First, the router looks for an IP Host table on the local router to perform this resolution – that’s what the “translating” word in the output is referring to. If there’s no IP Host table or the IP Host table doesn’t contain an entry for what you typed, the router will send a broadcast in an attempt to resolve this name through a remote DNS server. To prevent this broadcast, enter the global command no ip domain-lookup. Of course, to use DNS to resolve hostnames, ip domain-lookup would have to be reenabled if it’s been turned off.

R2#contin

Translating "contin"...domain server (255.255.255.255)

% Unknown command or computer name, or unable to find computer address

A command is mistyped as “contin”. The Cisco router’s default behavior is to resolve this entry locally via an IP Host table, which isn't present on the router. A broadcast is then sent out to find a DNS server to perform the name resolution. The DNS lookup attempt must time out before the configuration can continue.

R2#conf t

R2(config)#no ip domain-lookup

R2#contin

Translating "contin"

% Unknown command or computer name, or unable to find computer address

With “no ip domain-lookup” configured, the router doesn’t attempt to find a remote DNS server. It sees there is no local resolution configured and almost immediately sends a message to the console that the name can’t be resolved.

R2#conf t

R2(config)#ip domain-lookup

R2(config)#ip name-server 10.1.1.1

R2#contin

Translating "contin"...domain server (10.1.1.1)

A DNS server is installed on the network with the IP address 10.1.1.1. DNS lookup is reenabled with the command ip domain-lookup, and the IP address of the DNS server is specified with the ip name-server command.

It's just that easy to tell a Cisco router exactly where the DNS server is!

About The Author Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage (thebryantadvantage.com), home of free CCNA and CCNP tutorials, The Ultimate CCNA Study Package, and Ultimate CCNP Study Packages. You can also join his RSS feed and visit his blog, which is updated several times daily with new Cisco certification articles, free tutorials, and daily CCNA / CCNP exam questions! Details are on the website. For a FREE copy of his latest e-books, “How To Pass The CCNA” and “How To Pass The CCNP”, visit the website and download your free copies. You can also get FREE CCNA and CCNP exam questions every day! Get your CCNA study guide from The Bryant Advantage! chris@thebryantadvantage.com