This tutorial explains contiguous and discontiguous networks in detail. Learn the differences between contiguous and discontiguous networks through examples.
What are the contiguous and discontiguous networks?
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 a network in which packets sent between at least one subnet must pass through subnets of a different network.
In simple terms, a network is considered a contiguous network when a host of the network can access any part of the same network without going outside the network. To access some other part of the network, if a host has to go through some different network then the network is considered as a discontiguous network.
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.
This network topology is created on Packet Tracer. You can download this network topology from here.
Link to download the contiguous network topology.
To download Packet Tracer, visit this page.
In this network topology, if a host wants to communicate or exchange data packets with other hosts of the same network, it does not need to cross the boundaries of the same network.
Let's understand how it works practically.
Suppose PC0 wants to communicate with PC3. PC0 creates a data packet for PC3. Since PC3 is not available in the local subnet, PC0 sends this packet to the default gateway (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, the 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 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 use the 'tracert' command.
The following image shows the output of the 'tracert' command from both PCs.
The term contiguous is network specific. It does not include subnets of other networks. In our example, we have three networks: 10.0.0.0, 20.0.0.0, and 192.168.1.0. They all are contiguous as long as their hosts can access other hosts of the same network without going outside of 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 that we used above. In this topology, subnets 10.2.0.0/24 and 20.1.0.0/24 are interchanged.
You can download this network topology from here.
In this network topology, if a host wants to communicate with other hosts of the same network, it has to go through another network's subnet. For example, if PC1 wants to communicate with PC7, it has to cross a subnet 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 the different network (192.168.1.0/24), the network (10.0.0.0/8) is considered as a discontiguous network.
The following image shows the output of the 'tracert' command from both PCs.
As you can see in the above image, 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 know whether a network is contiguous or not, check its subnets.
If all subnets of a network are organized in such a way 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 of the same network that belongs to 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.
What type of network topology should you use?
When designing a network, you should always arrange subnets in a contiguous way. Contiguous subnets have several advantages over discontiguous subnets. If subnets are contiguous, routing protocols summarize them before advertising. I will explain this feature through examples in the next article.
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