View Full Version : Modem IP# vs Router IP#

05-20-07, 08:41 PM
Ok this is a bit strange and not sure if I believe the comcast CSR or not cause frankly I've had nothing but problems with packetloss for the last month.

However, when I connect a PC just to the modem(SB 5120) I see a xxx.xxx.xxx.101 address

When I connect back to the router (DGL-4300) I see a xxx.xxx.xxx.195 ???

I'm not really understanding why its picking up two different IP#'s when the modem was never reset?

Girl on the phone or CSR said its cause of the two different machines but I don't believe that. I'm thinking this can't be right and can't be good for whatever reason.

So far I've reset the router(DGL-4300) with a power cycle, holding in reset button for 20seconds then plugging back in.. resetting it to default in the controls, releasing the DHCP and renewing with no luck the router still shows the xxx.xxx.xxx.195 addy and the modem shows the .101

Anyone got any suggestions or explainations?

05-20-07, 11:57 PM
i dunno, but it's plausible that they assign ips based on an algorithm that takes into account the mac address of the machine but i'd assume they'd be dealing w/ the modem's mac addy, not your box's..i dunno. why does it matter?

05-21-07, 04:14 AM
Yes- they're tied to the MAC address of whatever PC or device is connected to them. That's why if you need to change your WAN IP, you usually have to configure your WAN port to spoof a different MAC address, otherwise it keeps the same IP- you can't change it. Most cable connections I've seen will not renew unless a device has been disconnected for an extended period- usually at least 8-12 hours, depending on how the iSP has it set up.

05-21-07, 08:49 AM
ok so according to yall it doesn't matter and won't hurt nothing.. well it matters cause I've been having packetloss for a month and I'm just making sure its Comcasts fault not mine and with taking that out it fixes that idea

05-21-07, 11:51 AM
Won't hurt anything...except for the PC plugged right into the modem with a public IP address...you risk exposing it.

Only thing it really can rule out is a faulty router.

DHCP services can hold an IP for a unique MAC for a set period of time. They aren't put back into the available pool until that time period is eclipsed.

05-21-07, 03:56 PM
Won't hurt anything...

yeah thats all I pretty much needed to know... and I only put the pc behind the modem only for a few mins to test it out to again rule out the router.. cause I'm doing what I can to ensure its not anything wrong with my systems or network in house.. its comcasts problem that is giving me a headache

05-22-07, 03:41 AM
Is it one of those 2-in-1 modems ?

05-22-07, 09:01 AM
Is it one of those 2-in-1 modems ?

no.. the modem is seperate Moto 5120
and the router i'm using is a DGL-4300