administrative distance depends on the type of routing protocol you are using. For example RIP uses hop count. OSPF uses bandwidth value. BGP uses policy ......
Thanks for your valuable comments.I just want to figure out that if the default administrative distance of RIP is 120..why 120 is chosen for RIP?2nd what is the unit of this administrative distance?this 120 will be in decimal units or in inches,meter etc,because some protocols select their shortest path based on shortest distance..