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Easy question for people that know about Ethernet and routers...

I have a router that connects to my internet with 100mbps ports only.

I want to plug in a gigabit router to one of its ethernet ports.

Of course this means that any device plugged into this secondary router would access the internet at a maximum of 100mbps only.

That is fine.

What I'd like to know is if the various computers plugged into this secondary router can communicate with themselves at gibabit speeds or it would it just be 100mbps? my guess is gigabit because they should be use local IP adresses of the secondary router (i.e. its 192.168.xxx.xxx), right?

  • When you say "secondary router", are you really talking about a router or merely a switch? That is, do you expect to separate the LAN into multiple subnets or not? – grawity Jul 5 '17 at 7:19
  • I didn't really think about it the main thing is that the devices on the "secondary" router need to communicate with each other, access the internet and a computer on the internet router needs to be able to access shared folders behind the secondary router, and possibly access some web pages being served from device behind the secondary router (but those served pages don't need to be able to go out to the internet) – Mikey Jul 5 '17 at 7:50
  • Then don't bother with routing! Get a simple switch. – grawity Jul 5 '17 at 9:46
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The key term is bottleneck. Imagine you have this network:

      PC1                      PC3                    PC5
       |                        |                      |
       |                        |                      |
       |                        |                      |
+--------------+        +--------------+        +--------------+
|   Gb Switch  |--------| 100Mb Switch |--------|   Gb Switch  |
+--------------+        +--------------+        +--------------+
       |                        |                      |
       |                        |                      |
       |                        |                      |
      PC2                      PC4                    PC6

Obviously PC1 and PC2 can communicate at gigabit speeds. As can PC5 and PC6.

It's probably fairly obvious that PC3 and PC4 will never be able to communicate faster than 100Mb.

But the bottleneck comes in when PC1 tries to talk to PC5. It has to go over a 100Mb link, and that now becomes the greatest possible speed (the bottleneck).

The maximum link speed between 2 machines is equal to the smallest speed in the link. As long as all devices are connected to a gigabit-capable device, they will have gigabit speed between them.

In your example, if it looks like this:

      PC1
       |
       |
       |
+--------------+        +--------------+
|   Gb Switch  |--------| 100Mb Modem  |-------- Internet!
+--------------+        +--------------+
       |
       |
       |
      PC2

Then as you said, the bottleneck only affects internet access (which is unlikely to be a factor).

  • great thanks, just wanted to be sure before I went ahead and ordered a gigabit router for this – Mikey Jul 5 '17 at 5:08
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    It should be mentioned that this is only complete when both hosts are within the same subnet (or otherwise were told to consider each other to be "on-link"), but packets which don't match an on-link route would go all the way through the router and back. Let's say someone has two subnets on the same LAN segment (silly, but happens) in which case hosts don't automatically know that the 'other' subnet is also on-link, so the router might become the bottleneck regardless. – grawity Jul 5 '17 at 7:17

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