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What is the correct term for a subnet "served" by a router?

cat pants

1/2/24, 9:31 PM

Imagine you have a router with a LAN port ip address 10.0.0.1. Assume the subnet is a /24. Assume no DHCP, just static ips.

You also have a server with the ip address 10.0.0.10 that is connected to the router.

First of all, I'm guessing that if you gave the server an ip not in that subnet, ie, 10.0.1.20, it would not be able to reach the router at all. Since routers are used to reach other subnets, it would need a router in order to reach the router, if that makes sense. That explains why servers must be in the same subnet as the router's LAN port.

So a router provides a routing service to a specific subnet, depending on how it's configured. What exactly is the proper term for what the router is doing for that subnet? "Provided"? "Served"? "Routed"?

For example, fill in the blank in this sentence: "Make sure to give that server an ip address that is in the subnet <fill in the blank> by the router."

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networking

joeqwerty

1/2/24, 11:29 PM

The router is routing for that network, hence the name router.

cat pants

1/3/24, 12:43 AM

But doesn't traffic for a single subnet not use a router? Hence the term 'routed/routing' makes no sense here.

joeqwerty

1/3/24, 1:07 AM

I guess I didn't completely understand your question. A router routes traffic for a network when that traffic needs to reach another network. For intra-network traffic between hosts in the same network, the router does nothing and that intra-network traffic does not transit the router.

cat pants

1/3/24, 1:39 AM

Understood. So what exactly is the router doing when it says to the server "if you're not on 10.0.0.0/24, you can't talk to me or any other server on that subnet"? What piece of hardware is choosing what subnet the server has to "live" in? Could you connect say 100 servers to a dumb switch, assign them random ips, and the servers with ips that happen to be on the same subnet could all talk to each other?

user5870571

1/3/24, 2:18 AM

The router isn't saying anything to 10.0.1.20. What systems are able to talk to each other at the subnet level is controlled by the subnet mask of the network. The hosts in the same subnet can communicate with one another.

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user5870571

1/2/24, 9:57 PM

The proper term for a router that is providing a route to another network, with a client device routing traffic through the router to reach another IP address in another network would be gateway. In order for traffic to leave the LAN and reach the 10.0.1.0/24 subnet, it needs to connect through another device; the other device is referred to as a gateway.

Most routers these days provide a number of abilities including, routing (dynamic (e.g. OSPF, BGP, etc.) or static. Your question appears to be asking what you would call a router which provides a route to a remote network for clients in its LAN.

So with your example, you might have a network that looks like this:

server (10.0.0.10 - Primary Subnet (10.0.0.0/24) -> Router (10.0.0.1 - Primary Subnet (10.0.0.0/24)

The Router might have a static route that looks like source destination gateway. In your example, that static route may look like this:

10.0.0.0 255.255.255.0 (source), 10.0.1.0 255.255.255.0 (destination), 10.0.0.1 (gateway).

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cat pants

1/3/24, 12:50 AM

I am not interested in traffic leaving the 10.0.0.0/24 subnet. I am only interested in what configuration needs to be done for the server to reach the router. I mentioned 10.0.1.20 as an example of a "bad ip choice" for the server, that if selected, would not allow it to reach the router. The router in my example is not acting as a router or gateway, but it does(?) have a preference for ip choice for the server. It only "allows" an ip choice of 10.0.0.0/24 for the server in this case. But what is that called? It's not routing or acting like a gateway here, as I'm only asking about one subnet.

Nikita Kipriyanov

1/3/24, 3:10 PM

@user5870571 L2 switches never perform ARP lookup. They don't do any lookups at all. They manage their internal MAC address tables, but that doesn't have anything to do with ARP tables and that doesn't involve any active lookups, just passive watching the traffic.

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