Multi LAN network does the main router to the internet need to be in its own subnet?

Question

I have to design a network where there are 4 within which are different number of VLANs with varying number of hosts per VLAN depending on the building.

My confusion lays in the main router, does it have to be in a subnet all of its own?

So say I have chosen a private IP range 172.16.0.0 / 23 which I am then subnetting accordingly, would I put my main router on 172.16.0.1 / 30 and start my my next subnet 172.16.2.1 and so forth?

Answer 1

No, you cannot keep the router in a separate subnet.

Here is why we need use a router,

Lets say You have a set of hosts and switches talking to each other. They are all part of the same LAN and hence interact at layer 2.

When they need to interact with the outside networks(Eg. Some server of the internet) They use the router.

Here is what the router does

The router needs to have one interface which is in the same subnet as that of your hosts. Remember: This is needed becuase the hosts can only directly talk to the devices which are in their own subnet, For anything else the traffic needs to be routed.

The other interface of the router is connected to a different subnet. This may be another private network.

The router knows that interface 1 is connected to subnet a and interface 2 is connected to subnet b. Hence it can facilitate communication between the networks.

These are called as connected routes

Now there are millions of subnets, ofcourse you cannot connect your router physically to all of them :)

Hence we use, routing protocols (Another discussion).

So, TL;DR for the very purpose of allowing communication you cannot put your router in different subnet

Answer 2

I may not be interpreting your question properly because there appears to be at least one word missing in your network description ...

I have to design a network where there are 4 <missing-word> within which are different number of VLANs with varying number of hosts per VLAN depending on the building.

If the missing-word is buildings, then I'll suggest the following answer assuming that the main router is behind a firewall, or at least provides none of the connectivity for the multiple VLANs that are mentioned.

The main router, the one with access to the Internet, will have at least two subnets per my interpretation of your problem definition. One subnet, either a /30 or /31, will be for the connection to the Internet Service Provider (ISP).

The second subnet, either a /30 or /31, will provide Layer 3 connectivity to a firewall or to another router that is providing access to the buildings and VLANs that you mention.

Note that the connection to the ISP typically has a Public IP address assigned to it either statically or via DHCP, unless the ISP is providing Network Address Translation (NAT) at their router.

If the ISP router is providing NAT, then you may need to supply the /30 or /31 subnet address for the connection between the main router and the ISP router, or the ISP can provide a Private IP address via DHCP for this connection.

If the ISP is NOT providing NAT, then the main router will receive a Public IP address from the ISP, and will have to provide NAT, since you're using Private IP address space, per RFC 1918, for your VLANs. The firewall, if you're using one, could also provide the NAT functionality.

There may be other solutions for this design, but the suggestions above should provide the information you need.

Answer 3

the main router, does it have to be in a subnet all of its own

No. Note that an interface is connected/assigned to a subnet. A router only makes sense when its interfaces are connected to two or more subnets. Only nodes connected to those subnets can use the router.

You can connect each VLAN to the router directly or you can have intermediate routers between the main router and the end nodes. The intermediate routers can each have their own subnet (or p2p link) to the main router or they can share a common one.

Answer 4

Firstly your question itself is not providing required information to answer your query !

1. How your network topology seems to be ..
2. Where your are creating a Vlans in router(inter-Vlan routing) or in layer3 switch (switch virtual interfàce).
3. Usages of subnet pools is totally depends on your business requirements and how topology is designed.
4. However your can create 4 different Vlan subnets from one supernet pool.