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Subnet Mask "Slippage"

By jb14808 ·
What could cause the subnet mask of a Wintel machine on a large network to "Slip" its subnet mask bits?
The machines in question will be assigned an IP address from the Win2K DHCP Server and work fine for a while but then the Subnet mask, nothing else, will "slip" from a 24 bit to an 8 bit Mask. It's happenned on various machines. It hasn't happened on our Macs or Cisco stuff. It only happens to Clients of the DHCP server and then, not all of them.

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Subnet Mask "Slippage"

by jb14808 In reply to Subnet Mask "Slippage"

AND... The problem is intermittent!

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Subnet Mask "Slippage"

by Abu_Danny In reply to Subnet Mask "Slippage"

Hi,
Depending on how often your DHCP is setup to check on connections and how your effected PC reply to it (Network Card Traffic Config) it could show as if these PCs are slower than the rest is replying back where the DHCP will think they belong to another subnet.
Also, are the one effected similar in anything else, like on same hub, switch ??

Hope this helps.

George

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Subnet Mask "Slippage"

by Shanghai Sam In reply to Subnet Mask "Slippage"

Please repost. All of the MAchines are on the same Subnet, for sake of argument lets say it 192.1.1.0 255.255.255.0 off of a Cisco 3524XL & a 3512XL with a gigE backbone.

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Subnet Mask "Slippage"

by turambar386 In reply to Subnet Mask "Slippage"

Hey jb. Need more info.

What OSs are the client machines?

What scope options is the DHCP server set to push out to clients?
Is the DHCP server on a different subnet than the clients? If so, does it have a different subnet mask?

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Subnet Mask "Slippage"

by Shanghai Sam In reply to Subnet Mask "Slippage"

Please repost and see other replies.
The Machines are Windoze95/98.
DHCP SERVER is set to All subnets Local and it pushes Address, time server, router, DNS, TCP TTL, IP TTL, MTU.

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Subnet Mask "Slippage"

by expertpc In reply to Subnet Mask "Slippage"

This is very odd behavior!

What IP address are you using? is it 169.x.x.x? The only thing I can think of (very far fetched) is that your lease is expiring and your clients can not find a DHCP server so they drop to APIPA mode and pick up a 169.x.x.x address.

I know...but I had to ask!

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Subnet Mask "Slippage"

by jb14808 In reply to Subnet Mask "Slippage"

YOu ain't kiddin'! It's Completely F^(<1NG CRAZY!! But I know it's not the DHCP SERVER or the IP scope. Lease times aren't expiring it happens to One machine WHILE BROWSING ACTIVELY!!Totally Strange.

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Subnet Mask "Slippage"

by expertpc In reply to Subnet Mask "Slippage"

Another far fetched alternetive could be that you have a rouge DHCP sever (non-2000) handing out a bad scope.

As you probably know, the DHCP cliet requests an update halfway through the lease period. If the client is on the otherside of a routerand you are using DHCP Relay or have an RFC 1542 capable router, then you might look around that segment for a rogue DHCP server that would respond quicker then the authorized one.

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Subnet Mask "Slippage"

by jb14808 In reply to Subnet Mask "Slippage"

Nope I check for that already. Good call on the Router possibility though.

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Subnet Mask "Slippage"

by A.C In reply to Subnet Mask "Slippage"

Not an answer as such, but is there any relationship between the IP addresses of the machines that are slipping (like they are all in a small range of IPs ?), are they on a particular hub/switch. Are you blocking any possible DCHP requests getting outside of your subnet (thats unlikely to be the problem if you say that the leases are not expiring)
I further question, if you renew the lease using winipcfg or ipconfig, do the masks get back to full 24 bit C class or remain as an effective (or should that read ineffective) A class. ?

A.C

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