router with working DHCP server

Show of hands? I do.

Actually, Windoze cheats and stores the IP address for (my guess) about 12 minutes. The typical cheap wireless router does not store DHCP leases in some kind of non-volatile cache. It would be nice, but it's not economical. Instead, on power up, it re-populates the ARP cache with sniffed MAC addresses, and then uses RARP to re-populate the corresponding IP addresses. With most Windoze clients, you can power off the router, go away for up to 12 minutes, and come back with the same IP addresses. After 12 minutes it's somewhat of a crap shoot. If you've ever had to deal with Windoze XP retaining a stale DHCP assigned IP address after roaming to a different access point, and having to run ipconfig /release; ipconfig /restore to get XP to grab a new IP address, you'll undestand how tenaciously XP wants to old onto IP addresses. That's not it's job, or IMHO a great idea, but I think MS does it in self-defense.

This is correct behavior for RARP. If the machine that previously owned an IP address is NOT present and accounted for when polled, the DHCP server expires the ARP cache entry (deletes the MAC address from the ARP table) and re-assigns the IP address. Think of it as a lease time of about 12 minutes to take care of reboots, but nothing else.

Incidentally, wireless ports are handled a bit differently due to roaming issues. For example, Orinoco AP-2000 and such access points will expire radios after only 5 minutes. One reason is that most AP's have a relatively small MAC address table. The cheapo AP's can handle perhaps 32 radios. AP-4000 perhaps 64. AP-1000 and AP-2000, I think are 254 (the maximum for a Class C netmask). Keeping these entries live for the typical 24 hr DHCP lease time is guaranteed to overflow the MAC address table in busy locations. In addition, you cannot roam between access points if an AP will not release your IP address. I have the various coffee shop AP's set to expire in 1 minute, which means that if you roam between local AP's, you've got perhaps 1 minute for the AP's to switch (thanks to the lack of a common roaming protocol).

If you really want a long term DHCP lease, I've been using Freesco:

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saves the DHCP lease table as a file and can easily recover from power cycles. However, I'm constantly running out of IP's after a month or so even with about 250 possible entries.

Longer is not better for wireless.

Reply to
Jeff Liebermann
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Does anyone have a broadband router with a working DHCP server?

By working, I mean a router that after power cycle, would remeber or figure out the IP addresses it gave out previously, or are currently in use on the network.

I had tried many routers and APs with DHCP servers before and all of them would forget the IP addresses they gave out, and give out duplicate IP address already in use.

One way to test your router is this (assuming the DHCP server's starting address is 100):

Connect two DHCP client machines to the router. Say machine A gets address *.*.*.100 and machine B gets *.*.*.101. Power cycle the router. Reboot machine B and see if it gets an IP address. If your router has a simple DHCP implemenation it will give out *.*.*.100 address which is still in use by machine A. So B will never get an IP address and machine A's console probably will report that there's a duplicate IP address on the network.

I would expect the DHCP server to at least remember the IP addresses it gave out for the duration of the lease.

Reply to

A bit more on the subject.

From RFC1541: (...) As a consistency check, the allocation mechanism may probe the reused address, e.g., with an ICMP echo request, before allocating the address, and the client will probe the newly received address, e.g., with ARP.

What this means is that if the DHCP server feels the need to reassign an IP address (due to overflow, IP exhaustion, or a reboot), it may ping probe for the previous lease holder of the IP address to see if it is available. Since you so cleverly disconnected the previous lease holder of the specific IP address involved, ping will fail, DHCP will declare that the address is available, and reassign the IP address to the next victim. In simpler terms, if nobody is home after a reboot, there's no requirement to restore a bunch of stale addresses.

Reply to
Jeff Liebermann

Uh, the D in DHCP means Dynamic, which means you cannot count on getting the same IPA-to-PC assignments. If it matters to one PC, give it a static IPA in the same subnet as the rest of the clients, and exclude that IPA from being handed out via DHCP; works for me.

Reply to
Bob Willard

The DLink 604 (and probably a bunch of other routers) allow you to assign an IP address to a MAC address, so every time a given device requests an IP address, it gets the same one. DLink calls it "Static DHCP".

Reply to
Hank Karl

Actually, WD's expectation is correct. Although DHCP is dynamic, it gives out a LEASE (with an expiration time) for an IP address. You are entitled to use that address without further communication with the DHCP server until the lease expires. The DHCP server ought to keep track of leases until they expire. But for

Reply to
Ron Bandes

Very good post Jeff. Appreciate all the info. About XP holding on to IP addresses: no kidding. :) Release/renew is not something you would expect a user to know he had to accomplish.


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