When you type something (at the user or privileged prompt) that the router doesn’t recognize as a command, it assumes that you want to telnet there. So it telnets there if it’s an IP address, or looks in the config for a host table if you typed a word. If the word can’t be resolved to an IP address via a host table, the router broadcasts for a DNS server. This is good if what you typed was an IP address or host name that you want to telnet to—it means you don’t have to type the word “telnet.” This is bad if you just mistyped a command. It ties up the router until the DNS request times out. You can turn off the DNS request with the command no ip domainlookup at the global config mode.
Example:Router#debag
Translating “debag”…domain server (255.255.255.255)
(255.255.255.255) Translating "debag"...domain server (255.255.255.255) % Unknown command or computer name, or unable to find computer address Router(config)#no ip domain-lookup Router(config)#^Z Router#debag Translating "debag" Translating "debag" % Unknown command or computer name, or unable to find computer address Source: Global Knowledge Training 
