Chapter 5 teaches you about subnets and subnet masks. The objectives important to this chapter are on page 5-1:
Concepts:The graphic and paragraph on page 5-2 give a good introduction to the concept of subnets. Suppose your company has six divisions or locations, and logically needs six networks. Logically, you would want to be assigned six different network addresses (such as 132.132.0.0). Suppose, however, that you either cannot get or cannot afford licenses for six networks. Then you take the one network address that you do have and create six subnets. (Six, by the way, is not a magic number, it is just an example.) Page 5-3 presents a list of reasons for creating subnets:
Subnetting works by borrowing bits from the host portion of an address, and using those bits to identify subsections of your network. The use of borrowed bits only works because of subnet masks. A subnet mask tells hosts on a network which bits in an address are network address bits and which bits are host address bits. It does it by the use of 1s and 0s. Consider the table below:
Network devices read a mask to learn how to interpret addresses. Address positions marked by 1s in a mask are considered network address positions. Address positions marked by 0s in a mask are considered host address positions. Another way of saying this is that certain address bits are considered to be network address bits and the rest are considered host address bits. The actual method used involves Boolean math, but understanding it is not critical to understanding or using the concept. When a device reads an actual IP address, the rule from the subnet mask is applied, and the device understands which bits are for what address. Page 5-6 presents a table of the subnet addresses possible when borrowing
1, 2, and 3 bits of the host address. Note the first example: by borrowing
one bit, two subnets are theoretically possible. However,
as a general rule, subnet numbers using all 1s and all 0s are not used,
so borrowing one bit will probably not yield any usable subnet addresses.
This is why the formula for number of subnets is the one given:
Note that the subnet masks above do not match the standard masks from the previous table. The standard masks are classful masks, because they match the intended use of class address schemes. The masks above are classless, because they do not match any network class. Your book refers to a network hierarchy using classless subnets as a classless subnet hierarchy. From page 5-8, be aware that routers on the Internet only use the network bits of an address for routing. Routers connecting subnets within a network must use the network, subnet, and host bits for routing. From the same page, remember that host addresses may be reused from one subnet to the next, but not within a subnet. Since most of the address work you are likely to do with subnets will
be with class C addresses, it is wise of Novell to discuss that
next. Starting on page 5-11, you see a subnet scheme for a class C network.
Obviously, you cannot use eight bits to define the subnet: you
only have eight bits to define a host address to begin with,
and you must use some of them for the host address. You must strike
a balance between how many subnets you need and how many
hosts you may put on each subnet. In this example, the administrator
has decided to borrow 3 bits from the host byte for subnets, leaving 5
bits for host addresses. The chart on page 5-12 shows the default (classful)
subnet masks for class A, B, and C networks, and what that mask would
look like if you borrowed from 1 through 8 bits for subnet addressing.
The chart on page 5-13 shows the number of subnets theoretically
possible by borrowing 1 through 7 bits, the number of hosts theoretically
possible for each of these subnets, and the masks used. To calculate these
values, use the formula above: To calculate the number of hosts possible for each subnet, do the same calculation, except that for the value of N, you use the TOTAL number of host bits available in the address. For instance, if this were a class B address, and you were borrowing 3 bits from the third byte for subnet addresses, the remaining 5 bits in the third byte and the 8 bits in the fourth byte would give you 13 as the value of N. Some methods call the exponent M when calculating the possible hosts, but the math is the same. Page 5-15 begins the topic of assigning subnet addresses. Each bullet on the page could use some discussion:
Planning for growth is addressed in greater detail on page 5-17. When assigning subnets and assigning hosts, you should watch the shared byte, the one whose bits are used for both purposes. In general, assign subnets starting with the left-most bits of the shared byte. Assign host addresses starting with the right-most bits of the shared byte. Do not use more bits than necessary. This will allow you to redefine the subnet mask later, should you have to assign more bits to the subnet, or more to the hosts due to network growth. If you do not follow this advice, you will probably have to reassign addresses when you change the subnet mask. Page 5-19 begins an example of calculating and assigning subnet addresses. Note that this example switches to the standard calculation I have given you above, subtracting 2 from the calculated powers of 2. We learn on page 5-20 that N stands for the number of bits masked for subnets, and M stands for the number of bits not masked. (Wouldn't M and U have been better choices? Oh, well...) Page 5-21 continues the example, becoming confusing in the process. The next step is to calculate the actual subnet addresses, that is, the addresses of the subnets themselves. The book refers to them as the first, second, etc. but the advice above tells us NOT to use them in this order. This step is for reference purposes. The method is this:
The balance of the chapter is taken up by exercises practicing these skills. It is recommended that you practice a lot, remembering that you will have no means to convert from decimal to binary (or binary to decimal) in the certification test except what you know how to do by hand. |