Who knows about subnet masks?

[Ronald_Jeremy]

A friend just got set up with Comcast with a static IP. The tech gave him a subnet mask of 255.255.255.252. I have never seen the last ?'octet'? of 252. I have always seen .0

I know the subnet mask limits what subnets can connect. Will this hinder him in any way? He will be needing to access this static IP from multiple places on the internet.

 

[JAG72]

A friend just got set up with Comcast with a static IP. The tech gave him a subnet mask of 255.255.255.252. I have never seen the last ?'octet'? of 252. I have always seen .0

I know the subnet mask limits what subnets can connect. Will this hinder him in any way? He will be needing to access this static IP from multiple places on the internet.

This will not hinder him in any way. They most likely do this to prevent him from using and spoofing other peoples IP addresses. Using this subnet only makes 4 IP addresses available for use.

 

[harshness]

He will be needing to access this static IP from multiple places on the internet.

The subnet mask is 252 because that's what limits him to his addresses. 252 ends with a binary 100 which gives him a range of three numbers 01(1), 10(2), 11(3).

As long as your friend doesn't violate the terms of the service agreement (no servers), he should be fine with this setup. Comcast views their static IP offerings almost exclusively as a way of providing secure VPN access that some government, finance and health care providers require of their employees working from home.

 

[harshness]

Using this subnet only makes 4 IP addresses available for his use.

Broadband providers often don't let you use the zero address.

 

[JAG72]

Broadband providers often don't let you use the zero address.

correct but using that subnet does allow for 4 host IP's on that subnet. If you notice I had remove "for his use" while you were creating your post.

 

[Smith P.]

By rule, the mask will allow to use only TWO IP.

Ending 00 will treated as network ID, ending 11 - as broadcast in such subnet.
This SM usually used for '1 User - 1 Host config' (Modem and router).

 

[diogen]

Will this hinder him in any way? He will be needing to access this static IP from multiple places on the internet.

No, it won't.
And it is in his own interest to keep the mask as told, although I don't think there is any crime in replacing the last 252 with a 0 on his end.

Let's say the last three digit number in his assigned static IP address is 130 (just as an example) with the corresponding mask 252.
In binaries it will look like this
10000010
11111100
Wherever there is a 1 in the bottom number, no changes in the top number are allowed. Therefore, this mask allows IPs with the last three digit number being
10000000 (=128)
10000001 (=129)
10000010 (=130)
10000011 (=131)
Any of the four numbers could've been the assigned IP last number (depends on Comcast policy).
Keeping the mask as tight as possible minimizes the "discovery" (e.g. master browser) the computer will do: in this case it will only look for 4 devices
(having the IPs above) on the local network, for the rest - it will send a request to the gateway.
I you change the mask last number to 0, it will try to find 254 local PCs/devices.

This way Comcast prevents you from attempting to connect to your neighbour's PC.

Diogen.

 

[Smith P.]

See previous post; last and first IP will be not treated as host IPs, but NetworkID and broadcast. ( If he is not using Cisco routers)

 

[Ronald_Jeremy]

Thanks for the replies guys. I just tested and connected to his network from another ISP fine. His WAN ip ends in .153. But you have answered my question.

One other unrelated question, is a switch capable of carrying 2 or more different networks? Say you have an 8 port switch, can you have 4 ports running on 10.0.0.X and 4 running on 192.168.1.x? No need for connectivity between subnets, just want to know if it is capable of routing the two different networks?

 

[haertig]

One other unrelated question, is a switch capable of carrying 2 or more different networks? Say you have an 8 port switch, can you have 4 ports running on 10.0.0.X and 4 running on 192.168.1.x? No need for connectivity between subnets, just want to know if it is capable of routing the two different networks?

There is a difference between a router and a switch. What you are probably talking about is the consumer stuff you buy at a local electronics store, which in reality is "a router integrated with a eight port switch". The router does the routing, the switch does the switching.

That doesn't answer your question.

I suppose you might be able to do what your want (why?) by setting the routers netmask to 0.0.0.0 if it allows that. Note that this does not give you a router talking to two different subnets, it effectively gives you "one REALLY big subnet" (the entire address space). I've never tried something like this - it may not even be allowable. It's not a normal thing to attempt.

 

[Ronald_Jeremy]

Yes, I understand there is the difference between a router and a switch. And that you can dumb down a router by turning off DHCP and make it a switch. I was just wondering if a SWITCH(ie. it says SWITCH on it) is capable of correctly handling more than one network. And I don't want to do it. I am trying to figure out an existing situation that someone else set up.

 

[diogen]

A consumer level unmanaged switch won't allow subnetting.
A consumer level router won't route private IP address ranges.

Diogen.

 

[mike123abc]

ATT (formerly SBC formerly SWB) gives out 3 bit static ips here, you can buy larger blocks for a one time fee if needed. This gives 8 addresses, 1 for broadcast, 1 for the gateway on their end, leaving 6 IP addresses left over.

Switches do not know about subnets. They tend to look at the MAC addresses and send the packets to the right destination port if known, or broadcast if if not. If you have a 4 port router with a built in switch as long as you do not use the uplink port it should work as a switch.

 

[John Kotches]

A consumer level unmanaged switch won't allow subnetting.
A consumer level router won't route private IP address ranges.

Diogen.

A switch is a layer 2 device, operating at the Ethernet level. It doesn't have any concerns with what IP networks are connected. In the past I've run SQUID and socks servers connected into a "consumer level unmanaged switch" with multiple networks.

A router is a layer 3 device, operating at the IP level.

Neither A "consumer level" nor a "commercial grade" router are allowed to route private IP addresses so long as they are passed strictly along private networks. My home router has a route between 192.168 and 10. all within my house.

What neither can do is show those addresses to the open internet.

 

[John Kotches]

Yes, I understand there is the difference between a router and a switch. And that you can dumb down a router by turning off DHCP and make it a switch. I was just wondering if a SWITCH(ie. it says SWITCH on it) is capable of correctly handling more than one network. And I don't want to do it. I am trying to figure out an existing situation that someone else set up.

There's a bit more than that going on to change a router functionality over to a switch functionality. All the DHCP server functionality of the consumer routers do is give IP addresses to a DHCP client requesting an address. That still leaves the routing functionality intact. You have to shut down the routing functionality

You can run DHCP with or without routing, just as you can route with or without DHCP functionality.

 

[John Kotches]

There is a difference between a router and a switch. What you are probably talking about is the consumer stuff you buy at a local electronics store, which in reality is "a router integrated with a eight port switch". The router does the routing, the switch does the switching.

That doesn't answer your question.

I suppose you might be able to do what your want (why?) by setting the routers netmask to 0.0.0.0 if it allows that. Note that this does not give you a router talking to two different subnets, it effectively gives you "one REALLY big subnet" (the entire address space). I've never tried something like this - it may not even be allowable. It's not a normal thing to attempt.

It would fail.

With a netmask of all 0s everything is on the local subnet and there's nowhere to route to.

 

[John Kotches]

No, it won't.
And it is in his own interest to keep the mask as told, although I don't think there is any crime in replacing the last 252 with a 0 on his end.

In general a network with a 255.255.255.252 (aka /30) subnet mask is constructed with the router on one IP address and the usable host on the other.

For argument's sake, we'll take 192.168.100.128/30

IP addresses on that network are
.128 (Network)
.129 (Usable)
.130 (Usable)
.131 (Broadcast)

We'll assign .129 to the broadband box/router and we'll assign .130 to the available host.

The problem is that all of the network routing tables beyond the router are probably going to be in a restricted VLAN and potentially with a firewall instance that blocks any traffic that isn't from the permitted subnet on that port.

Or let me put it to you another way; it will likely fail to access any resources outside the house.

You need NAT (which is easily handled by the typical consumer router/switch) to get more than one usable machine on that network.


lots of other stuff deleted

Wherever there is a 1 in the bottom number, no changes in the top number are allowed. Therefore, this mask allows IPs with the last three digit number being
10000000 (=128)
10000001 (=129)
10000010 (=130)
10000011 (=131)
Any of the four numbers could've been the assigned IP last number (depends on Comcast policy).

No, 128 is the network, 131 is the broadcast address for the network, only 129 and 130 are assignable.

 

[diogen]

Ending 00 will treated as network ID, ending 11 - as broadcast in such subnet.

No, 128 is the network, 131 is the broadcast address for the network, only 129 and 130 are assignable.

I had the impression this is not true anymore with modern network equipment...

Neither A "consumer level" nor a "commercial grade" router are allowed to route private IP addresses...

Yes. But some Ciscos can be easily made do that...
Not sure how third party firmware (e.g. DD-WRT, Sveasoft) treat them. Stock Linksys will drop them.

Diogen.

 

[HokieEngineer]

A .252 or /30 subnet mask is commonly used for point to point IP links, like Psmith said. I believe you can use the first and last address for hosts (most commonly referred to as network and broadcast) although you will run the risk of incompatability with some hardware. So the general idea is to leave those reserved.

Comcast most likely does this so the customer cannot use more than one public ip address.

 

[Smith P.]

I had the impression this is not true anymore with modern network equipment...
<skip>Diogen.

It's hard to discuss 'hear-say' knowledge - after providing facts we could continue.