subneting problem (VLSM)

[mansour alazzi]

when i did subneting (VLSM) of may network id 192.168.1.0 first i make a subnetwork with CDIR /30 to connect two routers one R1 on 192.168.1.1 over interface G0/0 and the second R2 on 192.168.1.2 over interface G0/0 and then when i make another subnetowrk's ID 192.168.1.4 with CDIR /26 for my LAN and give the R1 192.168.1.5 over interface g0/1 but the 192.168.1.5 give me overlab with interface g0/0 on the same router

how it can be overlab the first NID is 192.168.1.0 and the second start at NID192.168.1.4  are  there any rules that i miss

or rules about how to arrange your subnetworks's IDs when you ues the VLSM 

[Kasun Bandara · Accepted solution]

Hi, @mansour alazzi 

if you take 192.168.1.4/26 as subnet, its subnet IP range will be 192.168.1.0 - 192.168.1.63. so its overlaps with 192.168.1.0/30 which have range 192.168.1.0-192.168.1.3. 

so suggestion is, to create big subnets from first IPs and build small subnets from remains in this kind of requirement. 

[Flavio Miranda · Accepted solution]

@mansour alazzi 

ip classless command. 

But, If you take a lager network range after a smaller network range they will overlap

 Take the larger range First, then the smaller after that.

[Joseph W. Doherty · Accepted solution]

". . . are there any rules that i miss . . ."

Yes, I suspect so.

Subnets address blocks all require starting on specific IP addresses.

A /30 must start on an IP address evenly divisible by 4.  A /26 must start on an IP address evenly divisible by 64.

Your /30 and /26, as specified, both start on the same .0, which is why they overlap.

As @Kasun Bandara and @Flavio Miranda explain, allocating subnets, back-to-back, in descending subnet sizes, if done correctly, will avoid accidentally overlapping subnets, but it's not required nor is often the best approach for long term IP subnet allocations