Topic: Networking question?

I had a service call yesterday from a client saying they could not print. I went to the account to check it out. Printing a config page, I noticed something strange (at least to me), the IP, Subnet and Gateway. Here is what it was set to :

IP : 152.8.190.14
SN : 255.255.255.252
GW : 152.8.190.13

I wanted to hook my laptop up to it via crossover cable so of course I had to change this. I did, and it worked fine. Printed like a champ.
I questioned the IT person on staff about the scheme and he told me that every computer in the building would grab an IP scheme like this. I tested it with my laptop set to DHCP and it grabbed :

IP : 152.8.172.14
SN : 255.255.255.252
GW : 152.8.172.13

Everyones computer in the building had a scheme where the first 3 octets were the same and the last was one off. One lady's computer was 177.192 with a gateway of 177.191.

My question is, why would you want your network set up like that? That would be such a freakin headache to troubleshoot when there's a problem.

Later,

C:\

No ideas on this ?

Later,

C:\

It is the strangest setup I have ever heard of, tbh.

The only thing I could remotely think of that might be applicable here is that either there are multiple networks being used in the same building (so that one needs to have seperate subnet masks) OR originally or at some point it was planned that there would be.

So for example if some IT Hiney set up a table of IPs and Subnet Masks, and that is the one that they decided to use and for some reason it became the main one (lot of things start out like this and end like this IMHO).

WebShaman | The keenest sorrow (and greatest truth) is to recognize ourselves as the sole cause of all our adversities.
- Sophocles

Unless they have several networks using the same infrastructure, that is seriously over-complicating things. I agree with Webshaman.

OK, well at least I'm not going crazy. I just could not for the life of me understand why you would want your network set up this way. This is a pretty large university and this is the only part on campus that is set up this way.

Oh well, thanks for the input

Later,

C:\

On second thoughts, I've seen that sort of scheme before. I could be wrong, but as they're not changing the default mask, it's not a multiple network issue. It's likely used to hamper cross-network transactions - an ugly way to stop people (read 'visitors') seeing each other's shares/files, etc., and make the network less open. As far as I know, even if you have a local switch/hub, you'll still see communication problems between machines so long as they use the gateway provided by DHCP.

In one or two situations like that, for temporary network installations (with shared printers, internet access, and file sharing/streaming), a cheap router can be very handy. Stick it between you and the network and use it as the DHCP server. Voila! Not necessarily always a good solution, and a bit naughty in rented office space with horrendously expensive internet connections (around London, I've seen people paying a ton per machine in pokey little offices) as it's technically theft regardless of their extortion.

If that's not it, I'm going to lay awake for days trying to figure it out...

(Edited by White Hawk on 07-23-2009 00:11)

There is one simple reason for it IIRC.

If the environment is really large (meaning that there are going to be more than 255 IPs generated within the Local Network), then it would make sense to also start using the 3rd Octal as well for generating IP addresses within the LAN.

Still doesn't make much sense to me to be using .252 as a Subnet Mask though but it certainly doesn't hurt I guess.

WebShaman | The keenest sorrow (and greatest truth) is to recognize ourselves as the sole cause of all our adversities.
- Sophocles

well my gripe was that every computer was something different.

Computer A
IP : 152.8.190.14
Subnet : 255.255.255.252
Gateway : 152.8.190.13


Computer B
IP : 152.8.177.14
Subnet : 255.255.255.252
Gateway : 152.8.177.13

Computer C
IP : 152.8.150.190
Subnet : 255.255.255.252
Gateway : 152.8.150.191

etc...etc...etc...

they were all over the place.

Later,

C:\

(Edited by CPrompt on 07-24-2009 21:43)

Guess we will have to "fall back" on WH's explanation then...

WebShaman | The keenest sorrow (and greatest truth) is to recognize ourselves as the sole cause of all our adversities.
- Sophocles

That's actually a fairly common practice. .252 gives you over 1000 useable addresses on a subnet. So, if your network is made up of more than 253 objects, then a .255 mask wouldn't really work if the plan was to keep them on the same subnet. You'd have to drop the subnet down to increase usable node addresses.

On the flip side, having a subnet at .252 (or lower) can result in a lot of broadcast traffic on one subnet, which might not be beneficial. But it's certainly useable and quite common.

I have one environment that uses .252, with ~20 servers, ~350 users, plus wireless access points, a dozen printers, 15 switches, video editing suites, firewalls, etc. All works extremely well.

The subnet isn't the issue - the seemingly unnecessarily large number of gateways is the unusual bit...

.....do you say DHCP assigns the IP addresses??

~Sig coming soon~

yeah, the biggest question that I have, is why you would want each computer to have that sort of IP / Gateway scheme where the gateway is one off of the IP. Just seems to be a very unnecessary way of doing things.

We did get the issue resolved though. They routed that line through a different VLAN where it didn't give out that kind of scheme. Put a static IP to the printer and poof! All worked like it should.

It could have been the age of the printer not allowing it to work correctly on that style of routing, but...that's a different issue

Later,

C:\

quote:

---

CPrompt said:

large university and this is the only part on campus that is set up this way

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It wouldn't happen to be the part of campus where the CS dept was, was it? Maybe it was a result from a student project. When I was at the univ, I ran the lab for 3 years, and the 'senior research projects' would use the live network we had running in the CS lab usually (a subnet from the univ network). Of course myself or the other person working with me would 'direct' them, and make sure things made sense, or at least "fix" everything when the semester was over.

Or... Yea, I don't know...

--

Any sufficiently advanced bug is indistinguishable from a feature.

Yeah, when i read this, i immediately thought, this is some kind of effed up honey pot or something?

The experiment theory echoes with my first idea.

(hi cprompt)

~this is not a signature~

I have something not too different on my network. We will have a lot of VOIP and remote teaching going on. Each main switch is also a router so it can route between the different classrooms on the switch but there are only 2 ports that will put you on a particular subnet at a time.

We do this to limit broadcast domains, keep people out of things they should not be in, and to manage network traffic. But we are also planning to scale this back and open it up later if it turns out it is not needed.

__________________________
Eagles get sucked into jet engines and weasels are oft maligned, but beavers just make nice hats.
WCG|FA@H

...but what about the potentially huge number of seemingly sequential gateways?

All those IP addresses have the same subnet, but are being routed, by the looks of it, through different (virtual?) gateways. It's having the same effect on local traffic as putting it all on separate subnets (so noses stay out of unrelated areas), but it just seems... odd.

(Edited by White Hawk on 08-22-2009 14:46)

quote:

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White Hawk said:

but it just seems... odd.

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yep. I am right there with ya. I kind of get what they are doing now that you mentioned it earlier about "keeping noses out of unrelated areas", but it seems a bit overkill

Later,

C:\