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If I have a network like:

Group1 -> Switch -> Switch -> Server (configured as gateway by DHCP)
Group2 -> Switch -----^

(Hope that makes sense...)

When computers in each group send packets to other computers in the same group (so same switch), will the packets go all the way through the second switch to the server and back again or will they go directly to each other via the first switch (that they're directly connected to)?

On a side note, if I have a 16 port gigabit switch, does that mean that every port can be fully utilized at once? So say I have 8 lots of two computers exchanging files to each other, will they all be able to run at gigabit speeds?

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closed as off-topic by David Schwartz, mdpc, Wesley, Andrew, EEAA Apr 17 at 5:22

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I assume by "group" you mean IP subnet. –  David Schwartz Dec 3 '12 at 7:31

2 Answers 2

up vote 4 down vote accepted

When computers in each group send packages to other computers in the same group (so same switch), will the packets go all the way through the second switch to the server and back again or will they go directly to each other via the first switch (that they're directly connected to)?

Generally speaking (and assuming this is 100% layer 2 traffic), they'll communicate directly with each other, through the shortest path possible. If two systems are connected to the same switch (and appropriate mappings are already in the switch's CAM table), no unicast traffic between the two systems will leave that switch.

On a side note, if I have a 16 port gigabit switch, does that mean that every port can be fully utilized at once?

No, not necessarily. There are some switches (very expensive ones) whose backplanes can support full utilization of all its ports, but the vast majority of switches cannot do this. You need to look at the specifications of your specific switch to see what its backplane will support.

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2  
The majority of cheap, small switches (5 to 8 ports) can do this, because they use single switching chips that are fully non-blocking. Once you get above 8 ports, multiple chips are needed, and the cheap switches don't have fast enough links between the switching chips. –  David Schwartz Dec 3 '12 at 7:30
    
Thanks! I'm just looking at setting up a network for a (~60 people) LAN party and thinking how terrible it would be for all that traffic to be wasted. –  user61832 Dec 3 '12 at 9:37
    
I guess 'gateway' means 'gateway to other networks'...? That had never occurred to me. God I'm dumb sometimes. –  user61832 Dec 3 '12 at 9:38

They will go directly in switch one - that is the WHOLE SENSE OF A SWITCH. It forwards packets only to those ports that they belong to.

If it would not be that way, you would need monster core switches as they see every packet on the network.

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