As you connect through your Internet Service Provider, your Local Area Network will behave as a - "subnetwork" - or - "subnet" - of the worldwide - "Internet". The design of the Internet permits subnets of various sizes, and the size of your own subnet determines the theoretical maximum number of computers or hardware servers that can communicate directly with one another without passing through your router. Most home or small office subnets accommodate as many as 256 distinct IP addresses. When any of your local computers sends information anywhere else, it will need to make a quick decision as to whether it can send it directly using your local ethernet hardware, or whether it needs to ask your router to send it to some other, remote subnet.
The size of your subnet is determined by your - "subnet mask". This concept was designed by engineers that were comfortable with the binary numbering system. As a result of this binary orientation, you will eventually notice that certain subnet mask values get used a lot. In particular, you will see these numbers used for most subnet masks:
255.255.255.255
255.255.255.252
255.255.255.0
If you speak binary, you'll recognize the special significance of those numbers and the reason why they can easily be interpreted to divide a 32-bit number into two different sections. If you don't speak binary, don't worry about it! You can satisfy almost every configuration question by just memorizing these three items of information:
1 of 3: Netmask 255.255.255.255 specifies just one single IP address for use by just one single computer or piece of network equipment. This netmask is used for certain special cases in which it's important to isolate one computer from all others.
2 of 3: Netmask 255.255.255.252 specifies a small subnet for a very basic router and just ONE computer. This is the way most Internet Service Providers will try to configure the external interface of your router for access from their - "Cable modem" - or - "DSL modem".
3 of 3: Netmask 255.255.255.0 specifies a subnet with 256 IP addresses. This is the way your local router will generally configure it's own local interface to establish the basis for IP on your LAN.
The DHCP protocol will work with your router's preprogrammed, - "default" - behavior to automatically configure all of the Netmask values that will ever be needed in your Local Area Network. If you are using the DHCP protocol everywhere, you should never need to specify a netmask.
Once this information is available to the computers on your network, they will use it in combination with their own IP address to determine whether any other IP address can be reached through local ethernet hardware, or whether it can only be reached through your router.
