Subnet Mask Question

[NoShoE46]

Ok guys. I have tried to do some searches on the Net about Subnet Masks and even tried reading it in Networking books. I just don't get it. I was wondering if someone can explain it in general so that maybe when I re-read the Net descriptions I can understand it.
Here is the description of what one is that I found on the net;

"What is a subnet mask?
Subnet mask is a 4 byte (32 bit) number used to identify the sub-network ID and the host ID from an IP address. All the hosts in a sub-network will have the same subnet mask. E.g. 255.255.255.0, 255.255.127.0, 255.255.0.0 "

How is a subnet mask formed?
For all the bits in the IP address which are used to represent the network ID, and the sub-network ID, the corresponding bits in the subnet mask will be one. For all the bits in the IP address which are used to represent the host ID, the corresponding bit in the subnet mask will be zero. For example, if the first two bytes of the IP address are used to represent the network (class B address) and the third byte is used to represent the sub-network, and the last byte is used to represent the host, then the subnet mask will be 255.255.255.0

I have seen Class C networks use the default 255.255.255.0 majority of the time. Why is this the case and when would I need to configure it to some other numbers??? I have seent the subnet mask calculator on the Net but I was wondering how you calculate it on your own.

Man, this stuff is confusing. I need all the help I can get. Any help is GREATLY appreciated.

 

[L3Guy]

Subnet masks have one function. It is used by the hosts and routers to determine if the route (IP address) is local or remote.
If you do a binary AND on the Network address and the subnet mask, you get the network that the route is destined for.
Applying the same mask to the interface IP address, gets the network you are on.
DO these match exactly?
If yes, the address (and host) is local.
If NO, the address (and host) is remote. send the packet to the gateway.(router)

If you notice, in binary, the subnet mask is all ones, then all zeros.
There are only 33 valid subnet masks.
0.0.0.0
128.0.0.0
192.0.0.0
224.0.0.0
240.0.0.0
248.0.0.0
252.0.0.0
254.0.0.0
255.0.0.0
255.128.0.0
... On and on until
255.255.255.252
255.255.255.255

if you do the binary, it comes easier.

. Why is this the case and when would I need to configure it to some other numbers???

The Class C default of 254 hosts is a very comfortable number to work with. Usually, its about the right number that can be grouped together before a router is required to control broadcasts.

I once proposed that a Phone support center have a 255.255.255.252 mask for all their workstations.
This (obviously to those in the know) required a Layer 3 switch. The subnet mask created a network that allowed the L3 SW., the PC, a broadcast address and all "network" address. What this did was to disallow DHCP servers, most routing protocols, and additional routers, etc. on someone?s desk.

In general, a subnet mask groups workstations so that a controlled number can communicate via broadcast. A designer?s choice kind of issue.

HTH

Doug

 

[kylef]

I think that it is MUCH easier to explain subnet masks in "slashful" notation:

The "old" way:
IP: 192.168.0.1
Mask: 255.255.255.0

The "new" way:
192.168.0.1/24

Meaning, that the first 24 bits of the IP address specify the network and subnet and the remaining 8 bits specify the host ID on that subnet.

The two notations are equivalent, but the latter is much more obvious. Especially with a more difficult mask. Take, for instance the following.

IP: 192.168.22.25
Mask: 255.255.255.128

could be written

192.168.22.25/25

In other words, it's the same mask as before with one extra subnet bit. An extra subnet bit means the network administrator has twice as many subnets he can use, but the number of possible hosts on that subnet has been halved.

 

[Omagh]

Look under 3Com's site for a tutorial on IP.
It is the finest tutorial that I have ever seen on the subject. Mind you if you are looking for a quick read then don't read it. It took a little while for it to sink in but boy what a beautiful treatment the author did on the subject.
Highly recommended.

Don't know if it is still there though.

 

[Santa]

You must also make sure you don't confuse Subnetting and Class notation.

Subnetting is the use of the subnet mask to group a series of numbers together as apart of a subnet of a class.

Classes are set in stone.

If the first bit is 0 it is a Class A address
If the first two bits are 10 it is a Class B address
If the first three bits are 110 it is a Class C address
If the first four bits are 1110 it is a Class D multicast address
If the first four bits are 1111 it is a Class E experimental address