Help understanding these home networking scenarios

Ok, I'm working on expanding my knowledge of networking beyond the bare basics of home networks & troubleshooting common connectivity problems in Windows + PC.

Here's a couple of scenarios involving my home network that I've been trying to understand, but to no avail – so far. Although the fact that I havent been able to do so yet isnt a big concern, since when when I do understand it, my intuitive understanding will increase significantly. Learning through hands on, applied scenarios is SO much more effective than struggling to memorize details in the style of most networking books Ive seen.

First scenario I'm trying to understand

- Home network: Typical wired cable service is what it is

Hardware setup in home:

-Cable modem/router which comes in from the ISPs local regional network, or whatever the correct term is.
-A single 5 port switch coming off the modem. All computers connected to this switch.
- DHCP + automatic obtaining of IP/DNS used on all computers
- All computers Win XP + SP 2, or 3.
- Nothing wireless – everything is standard ol' cat5

What I'm trying to understand

There are 4 computers connected to the switch. 3 of them have the following specifications

1st comp: IP assigned by DHCP = xx.xxx.248.69 | Subnet mask: 255.255.254.0 | Default Gateway: xx.xx.248.1
2nd comp: IP assigned by DHCP = xx.xxx.248.136 | Subnet mask: 255.255.254.0 | Default Gateway: xx.xx.248.1
3rd comp: IP assigned by DHCP = xx.xxx.248.87 | Subnet mask: 255.255.254.0 | Default Gateway: xx.xx.248.1

Here's what I dont understand

4th comp: IP assigned by DHCP = xx.xxx.251.155 | Subnet mask: 255.255.254.0 | Default Gateway: xx.xx.250.1

All 4 comps are on the same subnet – what is causing the 4th comp's IP to be 251.155, instead of 248.155 – in line with the others.
How is the default gateway different – they are all connected to the same modem (which would be the typical default gateway in this scenario correct?)

I've tried various things from the Windows end of it, to see if that was cause. I've double and triple checked that all of the settings in network connections + internet settings.


Second scenario I'm trying to understand

Occasionally, I bring computers home to do some work on them. These computers were not experiencing any connectivity issues prior to this scenario described below.

Typical scenario has been this:


- Connect computer to my own LAN to download updates, etc. No connectivity. Proceed to verify that it is not any of the hardware in my own home network. It never is. Proceed to check common things which I'm aware of, which include: NIC drivers, NIC card integrity (by directly connecting it to another computer, and accessing a shared drive on the other computer), verifying that DHCP is enabled, that internet options -> LAN settings are set to automatic obtain, and no proxy settings set. TCP/IP settings under LAN properties – set to obtain settings automatically.

- Ping 127.0.0.1 to verify tcp/ip stack working. Sometimes ill reset the stack via using netsh int ip reset, just to see if somehow that changed it. Expectedly, it didnt.

- Ipconfig /release + renew typically results in "Cannot contact DHCP… timed out, blah blah". Restarting computer results in ipconfig being reset to a default 169.254.xxx.xxx

- No default gateway detected/cannot ping default gateway
- Manual pinging of DHCP address fails – unexpectedly – hey you gotta try everything when youre trying to learn about it…
- Any software firewalls are disabled, just in case this might be a factor.

Now here are the two kickers to me which leave me scratching my head

- I manually set IP/subnet/Gateway to my local specifications, and wallah – everything works. Change it back to automatic, goes right back.

- I take this computer, with all its settings set to automatically obtain, hook it into my wired LAN at work (same ISP), and every time it works. There is nothing non-standard about the configurations in my home network, that I am aware of. My laptops used in my home network have no issues when taking them elsewhere – I.e. the computers on my home network dont have some non-standard configuration that is specific to some non-standard settings in my home network – or so it would seem….

So it would seem to me that: My home network configuration is by the book standard, the network at my work is a standard setup, my own computers are standard – no proxies or anything – and all of the problem computers have been standard, in all of the areas that Im aware of.

Questions

- What other areas should I be looking at to understand why the visiting computer doesnt work on my home network with automatic settings, but does work with manual settings.

- What should I be looking at to understand why my computers have no problem contacting the local DHCP server, but the visiting computers – with apparently the same standard settings – and fully functioning hardware + TCP/ip stack + DHCP enabled in windows…… cannot?

Can you tell me who your ISP is and what make/model of modem you have?

You stated that you have a "standard" configuration at home, but this actually is quite "non-standard." It is so non-standard I have NEVER seen anything like it for a home network on a standard home based cable modem/router.

In order to understand your IP addresses you need to understand subnetting. By doing some basic binary math your fourth computer, with the IP address of xxx.xxx.251.155 is actually NOT on the same subnet as your other three computers. This is evidenced by the totally different gateway. It required a different gateway address because the gateway has to be on the same subnet as itself. Your other three computers use a different gateway address that is on the same subnet as theirselves.

Home based modems/routers should be giving out PRIVATE IP addresses that are NON-ROUTABLE over the internet. It appears that yours is giving out public IP addresses in one way or another rather they are accessible from the internet directly or not. To top it off, it is apparent that it actually is giving out different subnets and gateway information. There is no other way to explain this than to say that your modem configuration is WAY out of whack and is undoubtedly the same reason why your other computers have trouble connecting on your network.

The only way to solve this problem is to fix the whacked out modem, but I'm very curious as who the ISP is and what the make/model is because it is obviously something very out of the ordinary.

For reference, non-routable IP addresses fall in the following ranges:
10.0.0.0 - 10.255.255.255
172.16.0.0 - 172.31.255.255
192.168.0.0 - 192.168.255.255

Normal information found on a home network will appear like this: (as in I don't think I have ever seen an instance where it wasn't like this)
IP: 192.168.x.x
Subnet: 255.255.255.0

As a follow up to this thread, a colleague of mine pointed out something I initially missed. It sounds as if your modem is not meant to have multiple machines hooked to it like it was a router. If it was, it would already have multiple ports on it to plug in multiple computers. It would also provide "private" IP addresses and have NAT capabilities. Instead, it likely has one single port, where it provides a single public IP address. If you want to share the connection with other computers you need to hook a router up to it, not a switch. By some chance you have managed to get a couple of machines to run like this, but it is clearly not how your modem is supposed to be hooked up.

The model of the modem is: Arris - "Touchstone Telephony Moden" – Model # TM402p/110 ISP: Bresnan Communications And yea, the IP addresses that get assigned via DHCP are not among the non-routable ranges that you listed. So in this case, is the default gateway NOT the cable modem? I.e. do some modems perform router-like functions, or no? If the gateway & the DHCP server were further out than my modem, then that would make a lot more sense to me. What is still confusing to me though, is why some computers work find, and others dont. For example, one of the computers that has always worked fine is a Dell Dimension 2400. Worked fine on my home connection right out of the box, and has worked fine right after a reformat as well. Yet, I've had 2 other Dimension 2400s that I couldnt get to work, even after doing the reformat that they were scheduled for. They even had the same NIC as the working 2400. So on paper, after the reformat, they should have been exactly the same. I'd also considered that maybe adding more than 4-5 computers to the switch could have been causing the issues, but Ive checked that out in the past by disconnecting other computers from the switch, so that it would be the same number of computers hooked up to it. Thanks for your input so far.