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New Member

Duplicate IP

Suppose a user in defined VLAN configure his PC with a same IP address of the VLAN "see below". What will be happen?

1)Is this VLAN became down

2) How can I prevent this

Router#sh run

int vlan 10

ip address 10.10.250.1 255.255.255.0

PC_IPCOnfig#10.10.250.1

3 REPLIES
New Member

Re: Duplicate IP

PCs in this subnet can not use the interface vlan as a gateway

New Member

Re: Duplicate IP

Asked this in case a user put it by mistake. What will be happened?

New Member

Re: Duplicate IP

Hi,

Your question is 'what would happen if a user configured the same ip address on their pc as the default gateway?'

You would have a network which would not function very well, since you would have two machines responding to the one IP. If you had a second workstation which was trying to send it's traffic via the default gateway, both the gateway and the PC would respond.

In practice you would find that a second PC would only have intermittant access, as some traffic is sent to the router and some to the first PC.

The first PC would presumably in this case have it's own IP as it's gateway. This is commonly used by ISPs with PPP. In this case, from memory, it would simply brodcast out for the default gateway. However seeing as the default gateway has the same IP address as the machine I don't think the PC would listen to anything from teh duplicate IP.

So, in summary, the first PC, with the duplicate address would not work. A Second PC would have intermittant access and the router would have console messages saying it had detected a duplicate IP address.

To stop this, you need to administer your workstations and ensure that users do not have rights to change their own IP address. You could also maybe look at using your router to be a DHCP server.

HTH,

Mark

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