Contiguous and Discontiguous Networks Explained

A contiguous network is a network in which packets sent between every pair of subnets pass through subnets of the same network. A discontiguous network is one in which packets sent between subnets must pass through subnets of a different network. In simple terms, a network is considered a contiguous network when a host can access any part of the network without leaving the network. If a host has to go through a different network to access another part of the network, the network is considered discontiguous.

Example of contiguous networks

The following image shows an example of contiguous networks. In this network topology: -

• Three network addresses are subnetted and used. These network addresses are: 10.0.0.0/8, 20.0.0.0/8, and 192.168.1.0/24.
• Two subnets, 10.1.0.0/24 and 10.2.0.0/24 of the network 10.0.0.0/8 are used on F0/0 and F0/1 interfaces of router R0, respectively.
• Two subnets, 20.1.0.0/24 and 20.2.0.0/24, of the network 20.0.0.0/8 are used on F0/0 and F0/1 interfaces of router R1, respectively.
• One subnet of the network 192.168.1.0/24 is used to connect R0's serial 0/0/0 to R1's serial 0/0/0.

Contiguous network topology

In this network topology, if a host wants to communicate with other hosts on the same network, it does not need to cross the network's boundaries. Let's understand it in detail.

Suppose PC0 wants to communicate with PC3. PC0 creates a data packet for PC3. Since PC3 is not on the local subnet, PC0 sends this packet to the default gateway (the F0/0 interface of router R0). The router R0 reads the destination network address of the incoming packet. Since the destination network address of the incoming packet is available on R0's F0/1 interface, router R0 forwards the incoming packet from the F0/1 interface. PC3 receives this packet from R0's F0/1 and processes it. In response, if PC3 sends a data packet back to PC1, the packet will go through the same route in reverse. The following image shows this process.

Throughout the entire communication, data packets exchanged between PC0 and PC3 do not take a route that is not related to the network 10.0.0.0. To verify this, you can print the path packets take. The 'tracert' command shows the path. The following image shows the 'tracert' output from both PCs.

The term contiguous is network-specific. It does not include subnets of other networks. This example network has three subnets: 10.0.0.0, 20.0.0.0, and 192.168.1.0. They are all contiguous as long as their hosts can access other hosts on the same network without leaving the network. The following image outlines these networks.

Example of discontiguous networks

The following image shows an example of discontiguous networks. Except for the location of two subnets, this is the same network topology used above. In this topology, subnets 10.2.0.0/24 and 20.1.0.0/24 are interchanged.

Discontiguous network topology

In this network topology, if a host wants to communicate with other hosts on the same network, it must go through another subnet. For example, if PC1 wants to communicate with PC7, it must cross a subnet boundary of a different network. Since PC1 and PC7 belong to two different subnets (10.1.0.0/24 and 10.2.0.0/24) of the same network (10.0.0.0/8) and to communicate, they have to cross a subnet (192.168.1.0/30) of a different network (192.168.1.0/24), the network (10.0.0.0/8) is considered a discontiguous network. The following image shows the 'tracert' output from both PCs.

As shown in the image above, packets exchanged between PC1 (10.1.0.3) and PC7 (10.2.0.3) take a route (192.168.1.1–192.168.1.2) that does not belong to their network (10.0.0.0).

How to know whether a network is contiguous or discontiguous?

To determine whether a network is contiguous, check its subnets. If all subnets of a network are organized so that their hosts can communicate with each other without going outside the network, the network is contiguous. If a host takes a route to communicate with other hosts on a different network, the network is discontiguous. The following image shows one contiguous network, 192.168.1.0, and two discontiguous networks: 10.0.0.0 and 20.0.0.0.

Conclusion

Understanding the distinction between contiguous and discontiguous networks is essential for designing efficient and reliable network architectures. Contiguous networks ensure that all communication between hosts remains within the same network, simplifying routing and reducing potential issues. In contrast, discontiguous networks require packets to traverse multiple networks, which can complicate routing and lead to connectivity issues. By carefully organizing subnets and analyzing network paths, you can optimize communication and avoid the challenges associated with discontiguous networks.