Does IP address matter when 2 hosts communicate through a switch?

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May 8th, 2009
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PC1 and PC2 are connected to switch 1 and switch1 is connected to router1. If the IP address on PC1 is misconfigured, it wouldn't be able to communicate with hosts on another network, but it should still be able to communicate with other hosts connected to the same switch(no Vlan setup)correct?

I had a question on a CCNA practice exam that had this setup and asked what hosts PC1 could communicate with. The answer was that PC1 could not communicate with any hosts. I can't understand why.

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jjchen978 Fri, 05/08/2009 - 07:57
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if the IP address on PC1 is in the same subnet as other PC on the same switch, it should be able to communicate with each other.

Giuseppe Larosa Fri, 05/08/2009 - 07:58
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Hello Ken,

the switch cannot act as a router.

if PC1 is misconfigured it is simply isolated and cannot talk with no other devices even those in the same vlan/broadcast domain.

if you move a printer with a fixed ip address to another switch port in another vlan no one can use it.

This is what happens with standard IPv4:

the out of context PC tries to ARP for its default gateway gets no answer and it is stuck there.

the other devices send their packet to the default gateway that will try to route the packet to the best ip subnet (longest match in ip routing table).

To be noted IPv6 has mechanisms to deal with this kind of scenario but it is a different matter.

Hope to help



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