Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 
You may experience some slow load times, errors, and slight inconsistencies. We ask for your patience as we finalize the launch. Thank you.

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. If you'd prefer to explore, try our test area to get started. And see here for current known issues.

New Member

placing a contigous block on the WAN and LAN interface

Just wondering why this occurs. I place an IP block on the serial interface, say When I place my LAN block,, it errors saying there is a conflict with Oddly, if I place 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

  • Other Network Infrastructure Subjects
New Member

Re: placing a contigous block on the WAN and LAN interface

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.