The Cisco IOS allows commands to be entered with only the first few letters of the actual command name. For instance, the IP address command could have been entered as ip add rather than ip address. The number of letters required before the command is recognized depends on the various other commands available at that command level. For instance, the interface configuration command ip a could mean ip address, ip accounting, or ip access-group. The interface configuration command ip ac could either be ip accounting or ip access-group. Once you have reached a point in the selling of the command that allows only one possible interpretation by the IOS, the IOS will accept the abbreviated command.
Source: Cisco Router Configuration & Troubleshooting by Mark Tripod, New Riders, 1999
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