placing a contigous block on the WAN and LAN interface

[tbiskner]

Just wondering why this occurs. I place an IP block on the serial interface, say 65.107.130.10/30. When I place my LAN block, 65.107.130.113/28, it errors saying there is a conflict with 65.107.130.8 Oddly, if I place 65.107.130.113/30 it takes it just fine. Is this just a glitch in the IOS or am I missing a subnetting rule?

p.s. any non-contigous block works fine

[svermill]

Start with your lesser mask. /28 boundaries are at .16, .32, etc. Then look at your longer mask. /30 boundaries occur at intervals of four. So you have the range of .0-.15 covered with a .13 /28 and overlapped with a .10 /30. Your /30 needs to start at .16 or higher or some multiple thereof. Basically, you subnet a subnet from shorter to longer masks. Once you subnet a subnet, the entire thing is occupied. Once you use an address in a subnet (e.g. .13 in a range of .0-.15), it cannot be further subnetted using VLSM.