Why does this work at all? SB5100 cable modem issues.

Post #225,261 by Another Scott 9/18/05 10:34:36 AM Reply	Why does this work at all? SB5100 cable modem issues. Hi, I had some DHCP issues in the KnoppMyth box [link\|http://z.iwethey.org/forums/render/content/show?contentid=225240\|I'm trying to set up], and it got me thinking. I have cable internet. My cable modem is a Motorola Surfboard 5100. I never did any configuration of it - it just worked. I use its Ethernet port, not the USB port. My network is 100BT and 802.11g configured to have 192.168.0.x addresses. A Buffalo G54 (802.11g) router/switch/WAP plugs into the 5100. It is configured to be at 192.168.0.1 and serves up DHCP addresses from 192.168.0.2 to 192.168.0.180 to the rest of the house. There is another Buffalo 802.11g wireless bridge/switch that is in the office (and 2 PCs and our PowerMac are wired to it). It is at 192.168.0.200. The default gateway for all of the PCs and the bridge is 192.168.0.1. A few weeks ago I was reading about the 5100 and saw a page that said that the configuration/status/etc. can be read by going to [link\|http://192.168\|http://192.168].100.1 And sure enough, up came the status page. But why on Earth is my network working? Isn't the 5100 on a different subnet? How do they manage to find each other? Is that the cause of the problems I frequently have (the network will quit working. I'll power cycle the 5100 and let it come back up. When it's done, the lights on the 5100 will be slowly flahsing "Send" and the "Online" light will be out. Sometimes playing with the coax wiring input to the 5100 (no splitters, use active splitter, use passive splitter) will get the network back quickly, but sometimes it takes a few hours for it to come back up. The problems seem more frequent when it rains. Am I correct in my current thinking that the 5100 cable modem should be on the same 192.168.0.x subnet as the rest of the home network? (Alternatively, all of the rest of the network should be on the same subnet as the 5100.) If not, why does this work? AFAIK, the subnet mask is 255.255.255.0 for all the connected devices. Could this multiple subnet stuff explain why the network goes down so frequently? Can the IP address of the 5100 be changed? I find no mention of it in the [link\|http://z.iwethey.org/forums/render/content/show?contentid=225240\|FAQ list] and the [link\|http://broadband.motorola.com/consumers/products/SB5101/downloads/SB5101_UG_EN.pdf\|manual] (72 page .pdf) I've been able to find. Assuming I can't change the IP address of the 5100: If I do change the rest of the network to be on the same subnet as the 5100, does it matter what I choose for the IP address of the DHCP server? The 5100 acts as a gateway, so presumably its address should be used for the "default gateway" setting on the other boxes. Correct? Or should the Buffalo DHCP server be the default gateway? Or does it matter? Will everything else automagically work if the Buffalo Bridge/WAP is still the DHCP server at 192.168.0.1? Bottom line: Should I make any changes to my existing setup? It's working fine except for the annoying WAN connection drops at the cable modem, and the DHCP issue with the KnoppMyth box. Changing the IP addresses is a hassle because Windows insists on being rebooted so often... Thank you very much. Cheers, Scott.
Post #225,265 by broomberg 9/18/05 10:46:35 AM Reply	Got the same cable modem. 192.168.100.1 For every one. Makes perfectly good sense. Your local gateway / router KNOWS that the .100 net is not "local". So it passes the packets to its next gateway, which happens to reside on the other end of the modem. Even if the local cable modem allows those packets through, the next step gateway at the ISP will NOT route them, they are illegal. The modem KNOWS that anything starting with 192.168.100.1 is for it, grabs them, and allows the local connection. The only way this whole scenario does not work is if you have a local .100 network, which means you 1st hop router won't pass the packets to the modem. And since the cable company will not support local end-users with home networks other than trivial connection test, it is not their problem. If you, on the other hand have setup a "non-standard" home network (whatever that means) with multilpe segments, etc, then you better KNOW what segments you assigned and what breaks what. Note: This is for local log viewing and checking, so it is not like it really hurts something if you can't get to it, unless the cable guy shows up, and then he will tell you to plug in into a single DHCP enabled pc anyway.
Post #225,266 by Another Scott 9/18/05 10:51:07 AM Reply	So I shouldn't change anything? Thanks.
Post #225,272 by broomberg 9/18/05 11:02:19 AM Reply	Change nothing It is working exacly as it is design to. The cable modem is not ON your network, it is all by its lonesome like it should be. You have pegged the downtime to bad weather. Next time it happens, go to the cable status logs, grab screen shots (or wget pulls) od the messages concerning communication failures. Actually, you might want to script a wget to pull them on a schedule. They only keep the last 'x' entries. You would then write a script to dedupe the overlaps. Then when you realize there is a LOT more downtime than your interactive usage notices, you can use the logs as proof to your cable company of how bad the service is, and force them to do a bit of rewiring.
Post #225,311 by Andrew Grygus 9/18/05 2:42:03 PM 9/18/05 3:03:52 PM Reply	The modem is just a bridge. The 192.168.100.1 address is for management access only and has no relationship to the Internet traffic. It was probably picked because .100 isn't generally used on small networks. I had a Cayman router configured as a bridge and it worked exactly that way. 192.168.200.222 was it's address so I could access it, check status and reprogram if required. This was completely independent of the Internet traffic which used a 192.168.200.98 gateway, a Linksys router that got its WAN side address and gateway from the ISP through the Cayman bridge. The Cayman cared not at all about the IP addresses, it just figured out what was on each side by MAC address, than found the IP address associated with those MAC address and bridged between the ISP and the Linksys. Any traffic inbound not addressed to the Linksys and any traffic outbound not addressed to the ISP gateway it just did not pass. [link\|http://www.aaxnet.com\|AAx] Edited by Andrew Grygus Sept. 18, 2005, 02:53:38 PM EDT Edited by Andrew Grygus Sept. 18, 2005, 03:03:52 PM EDT Expand All History

Welcome to IWETHEY!