auto-summarization...

from the horse's  mouth,
http://www.cisco.com/en/US/docs/ios/12_2/iproute/command/reference/1rfeigrp.html#wp1017389
Route summarization reduces the amount of routing information in the routing tables.

By default, Border Gateway Protocol (BGP) does not accept subnets redistributed from an Interior Gateway Protocol (IGP). To advertise and carry subnet routes in BGP, use an explicit network command or the no auto-summary command. If you disable automatic summarization and have not entered a network command, you will not advertise network routes for networks with subnet routes unless they contain a summary route.
Enhanced Interior Gateway Routing Protocol (EIGRP) summary routes are given an administrative distance value of 5. You cannot configure this value.
Routing Information Protocol (RIP) Version 1 always uses automatic summarization. If you are using RIP Version 2, you can turn off automatic summarization by specifying the no auto-summary command. Disable automatic summarization if you must perform routing between disconnected (discontiguous) subnets. When automatic summarization is off, subnets are advertised.
Examples
The following example disables automatic summarization for EIGRP process 1:
router eigrp 1
no auto-summary

I've seen test questions that give a range of 4 IP addresses with slash notation, and ask that you choose the best summary route or rather the route that the protocol will select...


ok 4 addresses in the RANGE, say class B
rfc1918 states the default slash notation for class B is /16
what power of 2 = our RANGE?  (2^2 = 4) the answer is 2
now, nice and easy, we have 4 addresses in the question's RANGE, there are 16 subnet bits in Class B,
subtract 2 from the default class (16 - 2 = 14) so apply this mask /14 to the first IP address in the question's range and you've solved the problem...

if we had 16 IP addresses in the question, again Class B, what would our power of 2 be?
4 (2^4 = 16)
what is 16 - 4?
apply /12 to the first address in the range...

breathe in...