When Does a DHCP Server Reuse an IP Address???

[ClearToLand]

[NOTE: This is a learning experience situation, NOT a Production Problem. The workaround solution is obvious - but, what is the logic behind the problem as currently defined...]

• I have a SMC 7004BR set up as a DHCP Server handing out IP addresses 192.168.0.1 through 192.168.0.10 to a local LAN with (just recently) over 10 PCs (though never powered up at the same time - yet...).
• I also recently bought some new NIC cards, thus some machines now have a different MAC address, but with the same name.
• The DHCP Client List in the 7004BR is currently "full" with 10 entries. BUT, some are "doubles" - same machine shows up twice with different MACs - consuming two IP addresses

When I just changed the NIC card on another machine, it failed to get an IP address from the DHCP Server. Not knowing if the new NIC was good or bad, I gave it a Static IP, and manually entered the gateway and DNS addresses. It worked fine.

1. Why didn't the SMC 7004BR DHCP Server reuse one of the IP addresses currently assigned to a PC that was NOT currently powered up?
2. What's the logic / formula / rules for when a DHCP Server will reuse an IP address?
3. Do DHCP Servers in SOHO Routers follow a different set of rules than, for example, a DHCP Server operating under Windows 2000 or NT?

 

[SaigonK]

When half the lease time amount is completed, the client will broadcast another DHCPREQUEST packet to the server and asks if it can have the same IP for the next 1/2 of the lease time.
If the server cant supply an unused IP address or if it cant supply one based upon the clients MAC address, then it sends a DHCPNAK packet saying :you cant have an IP.

You are then free to try again.

If your router was setup to doll out 5 IP's only, and all five have a lease time of 3 hours and you use them all, they wont be dolled out to a sixth system until one of them is free.

 

[gunrunnerjohn]

There is no reason to restrict the DHCP address pool to the number of machines you have installed, and it frequently leads to problems as you have discovered. The reason the default address pool in your router was set to 100 addresses was to avoid just this scenario.

 

[SaigonK]

Originally posted by: gunrunnerjohn
There is no reason to restrict the DHCP address pool to the number of machines you have installed, and it frequently leads to problems as you have discovered. The reason the default address pool in your router was set to 100 addresses was to avoid just this scenario.

if you intend to have desktop systems on your lan, give them static IP's and turn off DHCP, it doesnt buy you anything to have it on.
If you intend to attach laptops with a cord or wireless to your LAn, then DHCP is your friend. Restrict the amount of IP;s available to restrict the amount of systems that can get connected.
Just a simple way to keep the connections to a minimum.

I ALWAYS restrict my pool to a bit more than the number of PC;s that will be connecting at one time or another.

 

[gunrunnerjohn]

Well, setting machine up statically just seems like more work for no gain, so I can't see the point. As far as restricting the DHCP pool, I fail to see any point in that as well. Can you give me any benefit to not using DHCP? I'm sure having a problem thinking of any...

 

[SaigonK]

Originally posted by: gunrunnerjohn
Well, setting machine up statically just seems like more work for no gain, so I can't see the point. As far as restricting the DHCP pool, I fail to see any point in that as well. Can you give me any benefit to not using DHCP? I'm sure having a problem thinking of any...

Set the IP once...your done. No running out (as this person clearly has).
Again, there would be an assumption that he might have visitors he doesnt want on his LAN.

 

[Nothinman]

Topic Title: When Does a DHCP Server Reuse an IP Address???

After the lease is expired, set the lease time really low if you need them recycled quickly. But realize you'll cause your DHCP clients to renew their IPs more quickly and this will generate a lot more network traffic but that shouldn't be a problem with that few machines.

Set the IP once...your done. No running out (as this person clearly has).

And when you change something like the DNS server IP's you have to run around and set them up again, with DHCP you only change the server and wait for the clients to renew their IP information. You still have the same IP range available, if you run out in either case (DHCP or static) it's a management problem.

Again, there would be an assumption that he might have visitors he doesnt want on his LAN.

DHCP doesn't have any authentication in the protocol, he'll need to put the security for that somewhere else. Perhaps disable all the ports except those in use, then he can reenable them when he wants to plug something in.

Do DHCP Servers in SOHO Routers follow a different set of rules than, for example, a DHCP Server operating under Windows 2000 or NT?

No, but they're usually less flexible meaning they probably come with hard coded default settings you can't change.

 

[SaigonK]

Originally posted by: Nothinman

Topic Title: When Does a DHCP Server Reuse an IP Address???

After the lease is expired, set the lease time really low if you need them recycled quickly. But realize you'll cause your DHCP clients to renew their IPs more quickly and this will generate a lot more network traffic but that shouldn't be a problem with that few machines.

Set the IP once...your done. No running out (as this person clearly has).

And when you change something like the DNS server IP's you have to run around and set them up again, with DHCP you only change the server and wait for the clients to renew their IP information. You still have the same IP range available, if you run out in either case (DHCP or static) it's a management problem.

Again, there would be an assumption that he might have visitors he doesnt want on his LAN.

DHCP doesn't have any authentication in the protocol, he'll need to put the security for that somewhere else. Perhaps disable all the ports except those in use, then he can reenable them when he wants to plug something in.

Do DHCP Servers in SOHO Routers follow a different set of rules than, for example, a DHCP Server operating under Windows 2000 or NT?

No, but they're usually less flexible meaning they probably come with hard coded default settings you can't change.

I would have to ask how many times in a small home or office network would you be changing DNS, if at all? Lets be realistic about this.
he isnt setting up a major office with 600 people, its 5 systems.

 

[SaigonK]

Originally posted by: Nothinman

Topic Title: When Does a DHCP Server Reuse an IP Address???

After the lease is expired, set the lease time really low if you need them recycled quickly. But realize you'll cause your DHCP clients to renew their IPs more quickly and this will generate a lot more network traffic but that shouldn't be a problem with that few machines.

Set the IP once...your done. No running out (as this person clearly has).

And when you change something like the DNS server IP's you have to run around and set them up again, with DHCP you only change the server and wait for the clients to renew their IP information. You still have the same IP range available, if you run out in either case (DHCP or static) it's a management problem.

Again, there would be an assumption that he might have visitors he doesnt want on his LAN.

DHCP doesn't have any authentication in the protocol, he'll need to put the security for that somewhere else. Perhaps disable all the ports except those in use, then he can reenable them when he wants to plug something in.

Do DHCP Servers in SOHO Routers follow a different set of rules than, for example, a DHCP Server operating under Windows 2000 or NT?

No, but they're usually less flexible meaning they probably come with hard coded default settings you can't change.

I would have to ask how many times in a small home or office network would you be changing DNS, if at all? Lets be realistic about this.
he isnt setting up a major office with 600 people, its 5 systems.

 

[SaigonK]

Originally posted by: Nothinman

Topic Title: When Does a DHCP Server Reuse an IP Address???

After the lease is expired, set the lease time really low if you need them recycled quickly. But realize you'll cause your DHCP clients to renew their IPs more quickly and this will generate a lot more network traffic but that shouldn't be a problem with that few machines.

Set the IP once...your done. No running out (as this person clearly has).

And when you change something like the DNS server IP's you have to run around and set them up again, with DHCP you only change the server and wait for the clients to renew their IP information. You still have the same IP range available, if you run out in either case (DHCP or static) it's a management problem.

Again, there would be an assumption that he might have visitors he doesnt want on his LAN.

DHCP doesn't have any authentication in the protocol, he'll need to put the security for that somewhere else. Perhaps disable all the ports except those in use, then he can reenable them when he wants to plug something in.

Do DHCP Servers in SOHO Routers follow a different set of rules than, for example, a DHCP Server operating under Windows 2000 or NT?

No, but they're usually less flexible meaning they probably come with hard coded default settings you can't change.

I would have to ask how many times in a small home or office network would you be changing DNS, if at all? Lets be realistic about this.
he isnt setting up a major office with 600 people, its 5 systems.

 

[Nothinman]

But that's a minor point, DHCP saves you time on setting up new machines, avoids typeos and makes it simpler to move a laptop between networks.

 

[gunrunnerjohn]

DHCP is easier to configure starting out, and easier to live with after the fact. Using the fact that you manually configured your network for security is foolhardy, that's not security! If someone has physical access to my network wiring, he has access to machines already running on the network.