Guidelines for Network Layer Addresses
The text offers a list of guidelines for making address assignments in your network on page 168. The author expands on many of them in the pages that follow, but not in the order of her advice, so let's do what we can with it.
Structured Model and Room for Growth
Assign an address to the enterprise network first, then subnet that address as need for each location, then subnet each of those subnets as needed for the divisions in the location. Plan each subnet to be larger than your immediate needs. Keep in mind that you will lose some addresses with each subnet you create, so don't make more than you need. Lose addresses? Remember that when you subdivide a subnet mask, you are creating a series of addresses which are assigned to the new subnets themselves and to their broadcast addresses, and which you cannot use as addresses for hosts. This means that you can run out of usable addresses or allocate addresses that will never be used if you start numbering without a plan. This is one reason that address schemes often use a ten-dot address as the starting address for the enterprise. It gives us the greatest flexibility for creating subnets and for those subnets to have large or small numbers of hosts.
Central Address Administration
|Value of Position (if a 1 is in it):||128||64||32||16||8||4||2||1|
Reading from left to right,
if the first bit (position 7, above) of the first octet (another word for a byte) is a
zero, that octet must
represent a number less than 128. This defines a class A address: the
first octet must be 127 or less. Consider it this way:
Private addresses are not
routable on the Internet. They are
perfectly routable on your own
private network. They can connect to the Internet when you use Network
Address Translation, which can be done on your own NAT server if you
have been assigned a public address range, or it can be done by your
ISP as discussed above.
The author spends half a page on the benefits of hierarchical
routing, then begins to explain it on page 179. Her explanation
includes a good argument that it simplifies finding a route to a
specific network. We should consider that a network can act like her
example of a ten digit phone number, in which the packets are passed up
to a router that can handle the first three digits, then to a router
that can handle the next three digits, then finally to a switch that
can handle the last four digits. IP addresses in the real world are not
assigned that way, but in our own private number scheme, they can
function just like that.
The text reminds us that IPv4 addresses are composed of two parts, a part that identifies a network and a part that identifies a host. The text refers to the network portion of the address as a prefix. The length of the prefix for any network is usually defined by a subnet mask that corresponds to Class A, B, or C addresses.
|Decimal Mask||Binary Mask|
When we actually create subnets from a classful network
address, we have to modify the subnet mask. By borrowing (stealing) one
bit, two subnets are theoretically possible. However, as a
general rule, subnet addresses using all 1s or all 0s are not used, so borrowing one bit will usually not
yield any usable subnet addresses. This is why the traditional
formula for number of usable subnets is:
2N - 2 = number of subnets (where N is the number of bits borrowed).
Assume we borrow two bits in each of the three classes above. Borrowed bits are shown in red in the resulting subnet mask numbers below:
|Decimal Mask||Binary Mask|
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. Classless Inter-Domain Routing (CIDR) is a router standard that allows the use of classless subnet masks. In the CIDR standard, IP addresses are followed by a slash and the decimal number of bits used in the network portion of the subnet mask.
An example of CIDR notation might be 10.12.78.0/24. This example shows a network address, 10.12.78.0. It is followed by a forward slash, and the number 24. The number 24 means that the subnet mask for addresses on this network uses 24 bits as network identifiers. This is the same thing as saying that the subnet mask for this network uses 3 bytes as network identifiers, which is the same thing as saying the subnet mask is 255.255.255.0. In the table below, the same network is described three ways. Each is telling us the same thing: IP addresses on this network hold network information in the first three bytes, and host information in the fourth.
|3 descriptions of the same network||Example|
|number of bytes that hold network bits||3|
When addresses are sent on networks that allow CIDR notation, they can include this helpful notation that is the equivalent of sending the subnet mask along with the address. This is particularly helpful in networks that use subnetting. In such cases, the number following the forward slash probably will not be a multiple of 8.
The text lists several routing protocols that support CIDR on page 181:
We are told that we should be able make use of route summarization (aggregation) to keep the size of
routing tables smaller. This is important because routers
advertise their tables to each other and because they search their
tables for routes. A smaller table means less traffic in
advertisements, and faster searches when those tables are used.
The concept of summarization is shown nicely on page 182. A branch office contains networks 172.16.0.0, 172.17.0.0, 172.18.0.0, and 172.19.0.0. The route to all of these can be summarized as 172.16.0.0/14. Why? Look at the chart on page 182 that shows the second octet of their addresses in binary. Their addresses vary only in their 15th and 16th bits. It is tempting to say that we could summarize these routes as /12, but that would also include 172.20.0.0 through 172.31.0.0 which may be in reserve for this office, or may be in use at another office.
The text does not explain something about this example. You can summarize this route as 172.16.0.0/14 because it meets the author's tests. Let's state them in another way:
is why the author's second example does not work. The problem
proposes that we summarize 192.168.32.0, 192.168.33.0, 192.168.34.0,
192.168.35.0, and 192.168.36.0 as 192.168.32/something. We would be
fine for some of the addresses, but the last one kills our plans. Look
at the binary version of the third byte in each case:
If we only cared about summarizing 32 through 35, we could say the leftmost six bits are identical in each case. That would lead us to say the summary route is 192.168.32.0/22. However, 192.168.36.0 breaks into the next bit. It now takes three bits to describe our subnets instead of two. So, what's wrong with saying we should summarize as 192.168.32.0/21? A lot. That summary would also include 192.168.37.0 through 192.168.39.0. Your summary must always include the numbers that would result in filling the subnet portion of the address with 1s, and all the addresses less than that, back to your summary address. If you are not including those subnets, your summary is invalid.
Let's move on to the naming section of the chapter on page 189. The text advises us that naming standards vary from one environment to another (e.g. WINS, Active Directory, Internet Domains). We are offered advice in the form of general suggestions that make sense:
Week 6 Assignment: Chapter 6