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

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. And see here for current known issues.

New Member

DHCP question


I am configuring my 3845 as a DHCP server, my first question is how can i prevent a client from having a static IP? i want to force them to get the IP through DHCP.

my second question is, if a client has disconnected, his ip and mac still shown when i do show ip dhcp binding, how to auto remove his ip and mac?



Re: DHCP question

A1. Client NIC card has to be configured to "get IP address automatically" instead to take static IP.

A2.By default, each IP address assigned by a DHCP server comes with a one-day lease, which is the amount of time that the address is valid. Once lease time expires, it's entry will automatically be removed.

If you wan to clear entry manually you can enter "clear ip dhcp binding "

HTH, rate if it does ...

Cisco Employee

Re: DHCP question

Hi Friend,

Answering your second question first where you want the ip binding with mac should be cleared. Default lease time period is one day which if you want you can lower down by configuring the lease time

lease {days [hours][minutes] | infinite}

To answer your first question on all widnown machine when you check the TCP/Ip settings there are 2 options (radio buttons) one for static ip address and another for dynamic ip address, so there you can select only dynamic ip address radio button but no doubt any one can change it manually so if you have some security configured on your windows 2000 server enviroment you can put sopme security that no general user can change and select that radio button for static ip adress.

I am not sure how that can be done on lilnux machine or some other operating system but for sure its operating system base settings and nothing to be done from router side.



*Pls rate helpfull post

New Member

Re: DHCP question


my second question was ansowered correctly, thank you both.

but for my first question, if a client has specified a static IP, is there anyway to deny his packets? i have no control over clients..

Cisco Employee

Re: DHCP question

Hi Baha,

Can you confirm your end users or clients are running which operating system?

This is client related settings and cannot be controlled by router so if you are running windows enviroment the administrator can disable the static ip address assignment section and also disable the radio button for those settings.



Re: DHCP question

if you know the IP of client which has been configured with static IP, then you can filter packets through access lists.

but as you said that you have no control over clients, I'm not sure how it will be managed even through ACL because it will create overload on administrator to keep changing ACL as IPs will keep on changing.

I'm not sure of any another way as I believe there is no way cisco device can come to know that whether IP is configured static or dynamically on the machine.

whatever we can do is either on clients side or at domain server to either restrict users change TCP/IP properties through group policy in windows server or to give basic privilege on the machine.