CCNA 200-301 Command Reference: ip domain-lookup

The above video is from my CCNA YouTube Channel.   Check it out once you’re done with this CCNA Command Reference on the ip domain lookup command.

Whether you use this command with or without the hyphen, the ip domain lookup command performs a simple but important task.   When it comes to practice labs or simulators, I’ve been disabling it for years with no ip domain lookup before I even start a lab.  It’s one of the four commands I run at the beginning of any lab:

R1(config)#no ip domain-lookup
R1(config)#no service timestamps
R1(config)#line con 0
R1(config-line)#logging synchronous
R1(config-line)#exec-timeout 0 0

You don’t have to leave a space between commands.  I do that here to make them easier to read.  I have a CCNA command reference for each one of those commands here on the site, so when you’re done here, check the other ones out!

CCNA Command:  service timestamps

CCNA Command:  logging synchronous

CCNA Command:  exec-timeout

The ip domain-lookup command is enabled by default, but you will not see it in the startup or running configuration when it’s on.   When negated, it will appear near the top of the config.   I’ll disable it now and then show you the top of the running config.   You can enable or disable this lookup with a hyphen in the command or you can leave the hyphen out.

R1(config)#no ip domain lookup

R1#show run
Building configuration...

Current configuration : 1430 bytes
! Last configuration change at 17:34:44 UTC Fri Feb 12 2021
version 15.1
no service timestamps debug uptime
no service timestamps log uptime
no service password-encryption
hostname R1
no aaa new-model
no network-clock-participate wic 3
no ipv6 cef
ip source-route
ip cef
no ip domain lookup

Now,  just what the heck does this command do?    If you enter something that isn’t an IOS command — in my case, that’s usually a misspelled IOS command — by default, the Cisco switch or router thinks you want that input sent to a DNS Server so it can be resolved to an IP address.   In short, the IOS thinks you’ve entered a remote device’s hostname.

In most lab situations, you won’t have a DNS Server location defined, so the device “helpfully” sends a broadcast to find one.   To show you what that looks like, I’ll reenable the lookup and then enter some authentic frontier gibberish at the command line.

R1(config)#ip domain lookup

Translating "sdfljkdf"...domain server (

That broadcast can take several minutes to resolve in a larger network.   In a lab, it’s likely just a few annoying seconds.  But what do humans tend to do when hitting a button once doesn’t give them the result they want?  They hit it some more!   Here’s what happens when I impatiently tapped ENTER three more times…

Translating "sdfljkdf"...domain server (
Translating "sdfljkdf"...domain server (

…. I sent out three more broadcasts, and now I have to wait for those to timeout before I can enter what I meant to enter in the first place.  Finally, I get the result shown here, and I’m plopped back at the prompt.

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

Admittedly, this is more of an annoyance than anything else, but if you’re a bad typist, you might want to disable this feature before you get to work in a lab.    After I do so, have a look at what happens when I enter more gibberish:

R1(config)#no ip domain lookup

Translating "sdfsdf"

% Bad IP address or host name
Translating "sdfsdf"

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

No broadcast for a DNS Server was performed because we disabled that feature with no ip domain lookup.

Here’s a CCNA Command Reference Video from my CCNA YouTube Channel that’ll show you this feature in action (and inaction).   Have a look and then check out the other command reference here on the site.  Thanks for reading and watching!

Chris Bryant

CCIE #12933

“The Computer Certification Bulldog”

Follow me on Twitter — @ccie12933.   See you there!  🙂