| IP Domain-lookup Commands. | | | | to Cisco our CCNA students will inevitably make |
| For people coming into Cisco networking for the | | | | typing mistakes, (although even the most dog |
| first time they are always surprised by a few of | | | | eared of network administrators will also making |
| the default command settings that are present on | | | | typing mistakes) |
| the Cisco routers and switches. | | | | To prevent the console from timing out every |
| These default command settings can be a real | | | | time a typo is issued it important to turn the "ip |
| pain if left at their default settings and unless they | | | | domain-lookup" off. At this point it is important to |
| are changed the CCNA student is generally going | | | | mention that only typos executed at the "user |
| to become increasingly frustrated. | | | | mode" and "privileged exec mode" will cause a |
| We continually practice the procedure of | | | | time-out to occur, if a typo is issued under any |
| instructing our CCNA students (and sometimes | | | | off the other commands prompts the Cisco |
| our CCNP students) that they must change the | | | | device will simply return an error message and |
| default settings in order to make their | | | | return the command prompt without any console |
| programming via the command line a lot more | | | | timeout. |
| fluid and without interruption. | | | | To turn off the default issue the following |
| The first of our house keeping commands is the | | | | command: |
| "ip domain-lookup" command. | | | | Note: The prompt is "router" but this command |
| The ip domain-lookup command is enabled by | | | | can also be executed on a switch. |
| default. The purpose of this command is to assist | | | | Router(config)#no ip domain-lookup |
| the administrator resolve names into ip addresses | | | | The emphasis is on the "no" argument. Execute |
| so they may telnet to remote devices. This | | | | the command by pressing the return key. At this |
| seems on the face of it a good setting, the issues | | | | point if a typo is executed at the "user mode" |
| are that the system does not understand the | | | | and "privileged exec mode" will not cause a |
| difference between a valid hostname and a typo. | | | | time-out to occur but instead the Cisco device will |
| In most cases if not all the system will interpret a | | | | try to interrupt a typo to an IP address using only |
| typo as a hostname and start to broadcast out | | | | the local host table held within the "ip host" table. |
| to DNS on the address of 255.255.255.255. | | | | Using the local "IP host" table is quite common, so |
| The broadcast itself is a minor nuisance on the | | | | rather than having to enter the full IP address of |
| network but the major issue that our CCNA | | | | the destination device that you have to telnet to |
| students find is that in a majority of times the | | | | you would simply enter the host name of the |
| console from which they are configuring the | | | | device and the Cisco device will lookup the name |
| system from be it tera-term, hyper terminal etc | | | | in the local host table for a matching name and |
| will lock up whilst it is broadcasting out for the | | | | then resolve the name to the associated IP |
| non-existent IP address for the typo. Being new | | | | address. |