Subnet Mask, Ip adresses and Primary Gateways

[original PM]

Hi All,

Probably a numpty network question so if you can see your way past the insults and post some relevant info it would be most appreciated

Is there any correlation between the IP address and Primary Gateway - it would seem that if the IP address is xx.xx.xx.xx then the primary gateway is always xx.xx.xx.01

And is the subnet mask always 255.255.255.0

I am refering mainly to standard home broadband not a larger company style network

Or are these items configurable but default to the above or is it just random?

[Pondlife]

My basic understanding is that your home router acts as a gateway to the outside world
And will typically use 192.168.0.1 or 192.168.0.254 for its gateway IP as the 192.x.x.x and 10.x.x.x. ranges are for private use.

The router will, unless configured otherwise, assign an IP address to anything that connects to it. The number range it uses will be configurable but is typically from about 192.168.0.60 to something like 192.168.0.253. You can usually set a device that you want to connect to have a static IP and it is advisable to use the 192.168.0.2 – .59 range to avoid conflicts.

Subnet mask is 255.255.255.0

Obviously these are the local settings on the router. Your ISP will be assigning your connection its own IP address which is used by the outside world.


Apologies if any of the above is bollux and needs correcting by someone who knows what they're talking about.

[doodab]

They are configurable. You do need to be careful with your choice of numbers though, there are specific ranges set aside for "private" use.

The subnet mask acts as a mask dividing an address into network and host parts. The network part corresponds to the most significant bits set to one, the host part to the least significant bits set to zero. So by changing the subnet mask you can facilitate having more or less hosts on a particular subnetwork. Any address on a network can be used for a gateway except the network address (all host bits 0) and the broadcast address (all host bits one). The "network" part will be the same for all the hosts on a network.

Have a google for CIDR.

[b0redom]

Hi All,

Probably a numpty network question so if you can see your way past the insults and post some relevant info it would be most appreciated

Is there any correlation between the IP address and Primary Gateway - it would seem that if the IP address is xx.xx.xx.xx then the primary gateway is always xx.xx.xx.01

And is the subnet mask always 255.255.255.0

I am refering mainly to standard home broadband not a larger company style network

Or are these items configurable but default to the above or is it just random?

192.168.x.x
and 10.x.x.x

are reserved for private use (ie these are non-routable on the Internet).

Subnet masks are basically used to say what hosts are on the physical bit of wire, so:

255.255.255.0 means 254 hosts (and 1 broadcast and 1 network address).

255.255.255.192 means 62 hosts (and 1 broadcast and 1 network address).

Generally for simplicity in home use, the IP addresses used are either 192.168.0.x / 255.255.255.0 or 192.168.1.x / 255.255.255.0

The final piece of the puzzle is gateway, ie how do you get to other hosts which are not on the same bit of wire. In home use, this is generally your router (192.168.0.1 or 192.168.1.1), but there's no reason you can't set that to somewhere in the middle of the assigned range. Indeed that's sometimes what corporates do (security through obscurity).

Your router will have a 2nd internet routable IP address and will transfer the requests from your LAN to the Internet (hence the name router).

Clear as mud?

[doodab]

172.16.x.x through 172.31.x.x are reserved for private use as well.

[original PM]

Hi All,

Thanks for you replies and yes clear as mud but I have learned something.

Cheers!

[Sysman]

There's also the autoconfiguration address range of 169.254.x.x

168.254.x.x is what gets used if no DHCP server is available and is used if for example you connect two or more systems together without a router. They'll work out a unique address for each computer amongst themselves so that you can set up an isolated network. I've tried this with Windows, Linux and OS X in a standalone network and it works.

The 192.168.x.x range is not recommended if you are setting up Virtual Private Networks (VPNs), purely because everyone and their dog (internet cafes etc) uses them and this makes conflicts more likely.

Having said that, if you ever need to get telephone support from your ISP, they might just ask you to set them to one in the 192.168.x.x range, especially if they supplied the router as part of a package. Fortunately Windows nowadays doesn't need a reboot when you change addresses.

The router address ending in .1 is merely a convention, but it's a handy one.

Many routers also allow you to specify a fixed address via DHCP for a given computer, so that it always gets the same address. This is handy for dealing with older operating systems.