Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 
Community Member

Auto IP address renewal through DHCP

One question. A laptop user is using an automatically assigned IP adddress by using a DHCP server. If this laptop is properly shut-down and then he/she has to go to work in another office that has a different subnet, does this user have to mannually do a "winipcfg" or "ipconfig" on his/her laptop in order for him/her to get an automatically new assigned IP address by the local DHCP, OR when this user just boot up his/her laptop, he/she will automatically get a new valid IP address from the local LAN subnet from the DHCP server ? Which one is it ? By the way, in this scenario, the lease for its previous IP address is not expired yet.

Cisco Employee

Re: Auto IP address renewal through DHCP

Once the PC/Laptop is rebooted, the TCP/IP stack will always negotiate the new ip address from the DHCP server automatically..You do not need to do winipcfg etc..for that.

Now if you shutdown the laptop which has a leased ip address and leased is not expired yet, the client is supposed to send a DHCPRELEASE message to the server so that the lease can be made available to other nodes. If the server doesn't hear from the client by the end of the lease, it marks the lease as non-renewed, and makes it available for other clients to use.

Community Member

Re: Auto IP address renewal through DHCP

So, let me see if I can get this straight !!! :-)

Part 1:

So, whenever a PC/laptop is newly rebooted, the local DHCP client will firstly check if it has already got an IP address assigned through some means. If not, it will go through the DHCPDISCOVER->DHCPOFFER->DHCPREQUEST->DHCPACK in order to get an IP address assigned.

But, if the TCP/IP stack knows that an existing IP address is already assigned, it will then firstly send out the first DHCPREQUEST to verify if its previously allocated IP address lease is still valid or not (ie to verify whether on the correct subnet or not), if correct, it will be responded with a DHCPACK. Or else, not correct, it will be responded with DHCPNAK. Then, it will somehow need to send out its second DHCPREQUEST to extend the existing IP address lease. Am I correct ?

Part 2:

And if the client no longer requires the use of its currently assigned network address (eg. the client is gracefully shut-down), the client sends a DHCPRELEASE message to the DHCP server. Upon receipt of a DHCPRELEASE message, the DHCP server marks the network address as currently not allocated. But this released IP address will not be indicated as one of the DHCP server's pool of available IP addresses. Hence, this released IP address will not be able to be relocated by the DHCP server for other client to use. But, if the same client bootups and attempts to verify its existing leased IP address, the same IP address will be allocated by the DHCP server. Am I corect ?

CreatePlease to create content