What on earth is a subnet mask and default gateway!?!?!?

Question

I was fiddling around today with XP and the IPCONFIG tool.. I found out that as well as an IP adress, I have a SUBNET MASK and a DEFAULT GATEWAY...

Ive heard these expressions before but cant work out what either of these codes are for! All I can work out is that the default gateway is something to do with routers...

...and whats with the number 255.255.255.0 ?? Is the subnet mask always that number?!?!

Please help!!!

Answer 1

The TCP/IP protocol is really just an addressing system that machines can understand. I am going to assume that you are talking about just one PC and are curious about the guts thereof.

The IP address is simply the unique address on the computer network you happen to be a part of. There are two types in terms of the public internet, routable and non-routable or private. This is also true of internal neworks in large organizations as well but on a much smaller scale.

Typically what you see on your individual PC, is a non-routable address that means nothing outside your house. This is where the Default gateway comes in. The Default Gateway is related to routing. Its job is to determine where the traffic sent from your PC is supposed to go and direct it to there.

Lets expand your network to two PC's in a work group. If those PC's have shared netowrk resources, like a picture file, when one PC wants to see, or copy a file from the other a request is sent via the network. The request is sent to the default gateway, which maitains a table of addresses that are on your network first. The Gateway compares requests to that table and if it sees that the request is for another PC on your network sends the traffic there.

What happens when the request if for shared resources (say a new security patch in a folder on Symantec's website) on a network outside your house? Well the router see's that the request is not bound for your network, and sends your request out allong your DSL/Cable/Dial-up connection to your ISP's servers who take it from there. That's why the thing is called a Router: it routes requests to the logical next step in the chain. As you can imagine, there are many routers on many networks between you and Symantec.

This is where the Subnet Mask comes in. The Subnet mask is used to distinguish between networks on the internet, or in large oroganizations. Basically there is a lot of math involved and it comes down to binary code but, simplified the common 255.255.255.0 subnet is used for what's called a Class C network. Simply put this means the first three octets if the IP address represents the network address and the last octet represents the address of the individual host (PC or printer or what ever has a network card) on that network. Class C has the most "networks" but the fewest possible "hosts." This is what I mean.

In the class C network 192.168.1.XXX , subnet mask 255.255.255.0, only the XXX part represents the host. There are 254 possible "hosts" on this network. "192.168.1.0" is not a valid address. The 255's tell the computer that the first three octets (the numbers separated by each period reading left to right) are reserved for the network address, and the last is reserved for host address.

But what happens if you need more than 254 "host" addresses on the same network? You go to a class B scheme. with a subnet mask of 255.255.0.0. This yeilds 65,000 host addresses on the same network. A Class A scheme of 255.0.0.0 only has 126 networks but close to 17 million host addresses.

I hope that answers your question.

But if you want to get deaper into the guts:

http://en.wikipedia.org/wiki/Classful_network

Answer 2

Ip Address Subnet Mask Gateway

Answer 3

This Site Might Help You.

RE:
What on earth is a subnet mask and default gateway!?!?!?
I was fiddling around today with XP and the IPCONFIG tool.. I found out that as well as an IP adress, I have a SUBNET MASK and a DEFAULT GATEWAY...

Ive heard these expressions before but cant work out what either of these codes are for! All I can work out is that the default gateway is...

Answer 4

Default Gateway IP Address: This is the IP address assigned to a router on a network. Within a single CIDR block, any computers connected to the network will route first through the gateway before traveling to any location outside of the network.

Subnet Mask: This is a bit value utilized in TCP/IP networks to determine the size of a CIDR block. Once the size of a CIDR block is known, routers and other devices can determine the Network and Broadcast IP addresses and ensure that IP traffic is routed correctly. Subnet masks are displayed in the same format as IP addresses: xxx.xxx.xxx.xxx. The size of a CIDR can be determined from a subnet mask by the following process: subtract 256 from each octet in a subnet mask and then multiply the resulting numbers. For example, the CIDR 255.255.252.0 becomes(255-256).(255-256).(252-256).(0-256). This becomes (-1)*(-1)*(-4)*(-256) which is equal to 1024.

Answer 5

255.255.255.0 is a subnet mask. Default gateway is the home for your IP. It is where broadcast are sent to contact your computer on a network.

Answer 6

click here for subnet mask

http://en.wikipedia.org/wiki/Subnet_mask


click here for default gateway

http://en.wikipedia.org/wiki/Default_gateway