Messinger MIS 4477 Subnetting

MIS 4477
Network and Security Infrastructure
JAKE MESSINGER (jake@uh.edu)

Subnetting

My Teaching Notes:

Subnetting is a method for getting the most out of the limited 32-bit IPv4 addressing space and reducing the size of the routing tables in a large internetwork. With any address class, subnetting provides a means of allocating a part of the host address space to network addresses, which lets you have more networks. You do not have to use the subnetting method to segment a network but if you did not, then the routers on your network connecting you to wan's and the internet would have to have an indivudual entry for each and every IP address allocated on your local network. Subnetting allows you to provide a range of "allowed" addresses that exist on your Local Network and addresses that are elsewhere through the use of a "Netmask".

IP Version 6 is still not in wide use. IPV4 is still the dominate network numbering scheme. It is important to be able to understand and know how to subnet a range of Network Addresses. Subnetting with netmasks (i.e. 255.255.255.0) is called Classful Networking. Subnetting with slash notation (i.e. 129.7.1.0/24) is called Classless Inter-Domain Routing. See the table in the Wiki referring to Class A, B and C networks.

Refer to the IPV4 Wiki to explain more on subnetting: http://en.wikipedia.org/wiki/IPv4_subnetting_reference

One of the most important points to remember in subnetting: (*) For routed subnets larger than /31 or /32, the number of available host addresses is usually reduced by two, namely the largest address, which is reserved as the broadcast address, and the smallest address, which identifies the network itself. Furthermore, when we are deciding what netmask to use to determine the size of our local network, we should take into consideration 1 additional IP to be used for your local router. Your PC calls this router's IP address the "Default Route" or "Gateway" address. This must be an IP address located within the SAME subnet as your PC or device. It has an internal or local IP address assigned but it also has another IP address assigne to it, usually on it's WAN port, either another ethernet port but many times this is a Cable Modem Port or DSL port. That WAN address is on a different network than your local LAN segment. Your PC doesn't need to know what that WAN address is. It only needs to know the Default Route/Gateway address. We usually use the first number in a range or sometimes the last number. It doesn't really matter, just as long as you tell your PC's and Devices specifically the local IP address of that router. So the bottom line is, you really lose 3 addresses off of your subnet that cannot be assigned to local PC's or devices. Example, a full Class C subnet (255.255.255.0 or a /24 network) is represented in binary as 11111111.11111111.11111111.00000000. Anywhere you see a 0 is the local net. The 1's are the Internet or WAN. There are 8 0's. So 2 to the 8th power is 256. You lose 3 of those addresses, so the actual number of ip addresses you can allocate to PCs and devices on a Class C subnet other than your gateway router itself is 256 - 3 or 253. See the Table in the Subnetting Tutorial.

Subnetting Tutorial (DO THIS TUTORIAL FIRST)
Subnetting Exercise

End of Lecture: Subnetting