Overlapping Subnets – the Problem
#CCENT and #CCNA testing, as well as real-life network engineering, requires that we find configuration and design errors in the network. Problems can occur when the design and configuration of all devices is perfect,with the root cause being some software or hardware problem with the devices in the network. However, configuration errors can be one of the more common sources of problems.
Today’s post discusses acts as a launch point to discuss and then practice for one class of design and configuration error: overlapping VLSM subnets. This post simply explains a little more about what I mean by overlapping subnets, as a setup for some upcoming exercises.
Every subnet can be described by its subnet ID and mask. That mask can be in either dotted decimal (DDN) format, or prefix format (sometimes called CIDR format).
Although we represent a subnet using its subnet ID/mask, the subnet actually consists of a range of addresses, beginning with the subnet ID (the lowest number), through the subnet’s broadcast address (the highest number). We just write the subnet ID and mask, and devices list the subnet ID and mask, to be brief. But we could just as easily describe a subnet with its range of addresses.
By design, subnets should not overlap. That is, the range of addresses in one subnet should be unique compared to all other subnets. In real networks, if two subnets overlap, when a router needs to send a packet to an IP address inside that range of overlapped addresses, the router may forward the packet to the wrong subnet.
Discovering When the Problem Exists
Next, consider a list of subnets on paper. It’s possible that the range of addresses in the subnets overlap, but it may not be obvious just looking at the numbers. For example, consider these two subnets:
At first glance, it may not be obvious as to whether these subnets overlap or not. As it turns out, these two subnets do overlap. How do you know? Calculate the range of addresses in each subnet, and then compare the two:
- 10.1.2.0/23: 10.1.2.0 – 10.1.3.255
- 10.1.3.248/30: 10.1.3.248 – 10.1.3.251
Note that all the addresses in the second subnet sit inside the range of addresses in the first subnet, so these two subnets overlap.
The ability to recognize when two subnets overlap – that is, the address range in the two subnets overlap – can be very useful on the ICND2 and CCNA exams, as well as the ROUTE and CCIE exams.
The process to info overlaps is simple: calculate the range of addresses in each subnet, and then compare the lists. Next post, we’ll do a few exercises to work on accuracy and speed!