How to retain a unique IP on a client with a static IP in a DHCP subnet?

Question

When I say DHCP subnet I mean that the clients are intended to receive their network configuration via DHCP, not statically.

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Let's observe the following situation:

I set up an IP address statically, and set the correct subnet and gateway. I'm presuming that, since the IP on the client is static, the client won't broadcast the DHCP discover. That is why the client doesn't know about the existence of the DHCP sever, and vice versa.

What is the mechanism which prevents the DHCP server to assign the address from the statically set client to some new client which asks for an Ip address?

Answer 1

In this scenario there's nothing that prevents IP conflicts from happening. For this exact situation there's an option to define reservations in the DHCP server, or you can set your static IP outside the DHCP scope.

Answer 2

http://www.ietf.org/rfc/rfc2131.txt

3.1.2

When allocating a new address, servers SHOULD check that the offered network address is not already in use; e.g., the server may probe the offered address with an ICMP Echo Request.

3.1.5

If the client detects that the address is already in use (e.g., through the use of ARP), the client MUST send a DHCPDECLINE message to the server and restarts the configuration process.

Answer 3

you have two option here.

1) Add the required IP in your DHCP reservation list, this will enable to get same IP always for the client from DHCP server. 2) Set a Static IP in your client machine , but the same time add the assigned IP in the exclusion list of the server.This is for restricting the server from assigning the same IP to any client.