DHCP

Dubb Rich

I work on the local navy base as a network tech. dynamic IP is supposed to change every time the pc connects to the net. DHCP is enabled, but every time I check the IP on this machine it is the same. I thought that with DHCP enabled the IP is dynamic, therefore, it is supposed to change? can anybody clarify?

Special_k21

My experience with DHCP
You are exactly correct. Your IP does change. But when a computer is shut down or rebooted, that IP is put back into a pool of avaliable IP numbers. The DHCP server assigns a machine the first avaliable number from that pool. So if your computer is 198.168.0.4 and reboots, its very possible for that computer to receive the same number.
I am no expert on this....am I on the right track?

BJG

It could also be that the network admin has allocated a reserved address for your pc for some reason, which would mean that the 1 IP address is reserved for your machine.

They could do that for all the tech support machines, or even reserve address pools for different depts. Considering that it is a navy base they could do something like that for monitoring and security.

RussS

Ok guys ... here's the deal.

DHCP is a fun and valuable tool, but it is also something that we need to learn inside and out so we can correctly diagnose problems. When a PC is shut down (W9x being prime candidates) it does not necessarily release the IP address and this can cause problems when trouble shooting (or moving a PC from one area to another with a different IP range) - In this case we use IPCFG or IPConfig to release & renew the IP address.
We must also address the issue of IP leases. Some administrators set this up so that it is done daily so as to aid with security (IE a trojan being installed on a machine with a known IP address - when it is changed this can make life difficult for the script kiddie hacker) and others set this for weeks or even months because they wish to keep the IP addresses constant for one reason or another. If I am not mistaken the standard setting for W2K server is 8 days.

I hope this clarifies things a little.

DamienM

I agree that this is a lease issue. Workstations don't necessarily release IP's when using DHCP.

Dubb Rich

thanks guys that really cleared my confusion

tokhss

couple more things to become aware of.

depending on the type of configuration, the IP could be bound the to the physical addy (mac addy) .. this could be let go depending on how long the lease time is or how long the device is disconnected. kinda how like ISP providers do it.. i know here at verizon, in the west, the using dhcp, the ip addy is bound the customers mac while the device is connected, if its not connected, after 4 hours you can say by to the ip addy.. in the east, the router (redback's ) hold the ip addy of the customer for 6 hours, connected or not.. if connected, you keep it, if not say byebye.

i say the server / router recorded your mac addy and now associates your mac to the same ip until a certian period of time passes, if your still not connected, meaning plugged in, then it will let go of the ip..

just a guess..

bellboy

RussS wrote:

Ok guys ... here's the deal.

DHCP is a fun and valuable tool, but it is also something that we need to learn inside and out so we can correctly diagnose problems. When a PC is shut down (W9x being prime candidates) it does not necessarily release the IP address and this can cause problems when trouble shooting (or moving a PC from one area to another with a different IP range) - In this case we use IPCFG or IPConfig to release & renew the IP address.
We must also address the issue of IP leases. Some administrators set this up so that it is done daily so as to aid with security (IE a trojan being installed on a machine with a known IP address - when it is changed this can make life difficult for the script kiddie hacker) and others set this for weeks or even months because they wish to keep the IP addresses constant for one reason or another. If I am not mistaken the standard setting for W2K server is 8 days.

I hope this clarifies things a little.

thanks

mrironleg

Dubb Rich wrote:

I work on the local navy base as a network tech. dynamic IP is supposed to change every time the pc connects to the net. DHCP is enabled, but every time I check the IP on this machine it is the same. I thought that with DHCP enabled the IP is dynamic, therefore, it is supposed to change? can anybody clarify?

DHCP is dynamic, but when setting up the DHCP server an administrator has the option of setting a lease time. Its the lease time that is providing you with the same IP every time you log on. For example if you leased an IP for 7 days, every time you logged on you would recive the same IP. If at the end of the lease you were not logged on and another machine needed your IP, its possible your IP would be different the next time you logged on.