IP ADDRESSING, THE BASICS
Support Bulletin: 6090
Sections contained in this document:
1. Structure of the IP Address
2. Networks and Hosts
3. Network Addressing and Host Addressing
4. IPAddress Classes and Related Number of Unique Addresses
5. Subnets and Subnet Masks
6. Gateways
1. STRUCTURE OF THE IP ADDRESS
It consists of four 8-bit bytes separated with a dot (.): xxxxxxxx.xxxxxxxx.xxxxxxxx.xxxxxxxx
Example of an actual address:
206.71.95.34
In binary, this would look like: 11001110.01000111.01011111.00100010
2. NETWORKS AND HOSTS
An internet consists of several individual Networks connected by bridges or routers. Each individual Network (also called a physical network) consists of a group of Hosts (computers, etc.) that are "directly connected" to each other via a common cable or common hub (or group of hubs).
It's important to clarify what
is meant by "directly connected". This involves cable
ports and hubs only. It does not include bridges, routers or
the I/O bus. For example, 2 Nics in the same computer are NOT
"directly connected" and therefore exist on separate
networks, even though they may be bridged or routed together
by the computer's operating system or other software. Similarly,
Hosts whose only common connection is a bridge or router, are
considered to be on separate networks. Like the 2 Nic example,
each cable port of the bridge or router are not "directly
connected" and therefore exist on separate networks. See
illustration below:
3. NETWORK ADDRESSING AND HOST ADDRESSING
Each Network and each Host requires its own identifying number, its IP Address.
The Network Address is a unique number, consisting of a special pattern, that is assigned to each Network (physical network). These addresses may not be used for Host addresses. Example:
network 1 above might be assigned: 206.71.95.0
network 2 above might be assigned: 195.153.0.0
network 3 above might be assigned: 147.0.0.0
(notice the trailing zeros in each example)
NOTE: If your network is connected
to the outside world, you must obtain your primary Network Address
from interNIC to avoid duplicating and conflicting with the IP
addresses of other organizations. If your network is isolated,
you may use any valid IP Network address.
The Host Address is a unique number assigned to
each individual device (network card, router, etc.). It will
be a sub-set of the Network Address to which it is physically
connected. As mentioned before, it is important to note that
Network Addresses may NOT be used for Host addresses. Example:
a host on network 1 above could be: 206.71.95.34 (but not 206.71.95.0)
a host on network 2 above could be: 195.153.125.10 (but not 195.153.0.0)
a host on network 3 above could
be: 147.100.150.32 (but not 147.0.0.0)
4. IP ADDRESS CLASSES AND RELATED NUMBER OF UNIQUE ADDRESSES
There are 3 classes of IP Network Addresses:
- Class A example: 147.0.0.0 (bytes 2,3,4 have a decimal value of zero)
- Class B example: 195.153.0.0 (bytes 3,4 have a decimal value of zero)
- Class C example: 206.71.95.0
(byte 4 has a decimal value of zero)
Compare bytes 2,3 and 4 in each
class. If a byte has a decimal value of zero, it means that all
8 bits have a value of zero, i.e., 0 decimal = 00000000 binary.
A byte that has a decimal value of zero has all 8 bits available for Host addressing; however, bytes that are non-zero are using all 8 bits for the network address and cannot be altered. For example:
195.153.0.0 (decimal) = 11000011.10011001.00000000.00000000 (binary)
(non zero)(non zero) (zero) (zero)
To help illustrate, let's arbitrarily use an "n" for each bit used for Network Addressing (can't be changed), and an "h" for each bit that can be used for Host Addressing. Our example above now looks like:
nnnnnnnn.nnnnnnnn.hhhhhhhh.hhhhhhhh
Each "h" byte has 256 possible combinations, i.e., 8 bits each with 2 possibilities (0 or 1) hence 2 and 8 and = 256. They range from 00000000 (0 decimal) to 11111111 ( 255 decimal). Since there are two such bytes in a class B Network Address, it has 65,536 possible addresses combinations (256x256). Class A Network addresses have 3 such bytes with over 16 million possible combinations; class C only has one such byte, hence only 256 possible addresses.
Not all of the possible combinations are a valid Host IP addresses. It's not legal to make all "h" bytes all 0 or all 255 ( bits all 0's or all 1's). All 0's is reserved for Network Addresses, all 1's is reserved for broadcasts to all Hosts on that network. Example:
195.153.0.0 (11000011.10011001.00000000.00000000) Network Addresses only
195.153.255.255 (11000011.10011001.11111111.11111111) Broadcast Address only
195.153.255.0 (11000011.10011001.11111111.00000000) OK for Host address
195.153.0.255 (11000011.10011001.00000000.11111111) OK for Host address
5. SUBNETS and SUBNET MASKS
What do you do if you need more than one IP Network. For example, if you have 2 network cards in the same server, they're on different physical networks and therefore need to be assigned different network numbers. Without different network numbers, the server doesn't know which adapter to communicate through.
One solution would be to request another Primary Network Number from interNIC, but that's not practical or necessary. Instead, you can use some of your available Host bits to define additional networks. Recall the class B example where the "n" bits are reserved for Network addressing and the "h" bits are available for Host addressing:
nnnnnnnn.nnnnnnnn.hhhhhhhh.hhhhhhhh
We may assign some of these "h" bits to be used to make additional Network addresses. This is called SUBNETTING. Here are the rules for making a Subnetwork (Subnet):
a) You may use all of the "h" bits except the last bit in byte 4.
b) They must be the "most significant bits" (MSB), i.e., the left-most group. Example:
OK nnnnnnnn.nnnnnnn.NNNhhhhh.hhhhhhhh (3 bits used, just for fun)
NOT nnnnnnnn.nnnnnnnn.hhhhhNNN.hhhhhhh
NOT nnnnnnnn.nnnnnnnn.hhhhhhhh.NNNhhhhh
Obviously, the more bits used for Subnets, the less bits available for Host addresses, but that's OK. You'll pick the class of Primary Network address that gives you the flexibility to closely fit your needs.
Let's make some Subnets out of the Primary Network address 206.71.95.0. In binary, this would look like:
11001110.01000111.01011111.0000000
Only byte 4 has a value of zero, so we only have 8 bits to play with. In order to decide how many bits we need for Network addressing, let's first think about how many Hosts and Networks we need. Let's arbitrarily pick 3 Networks each with 50 Hosts as our requirement. Let's consider some possibilities. Since bytes 1,2 & 3 can't be changed, let's only look at byte 4:
a) .nhhhhhhh gives 2 Subnets (21) each with 128 Hosts (27) not enough Subnets
b) .nnhhhhhh gives 4 Subnets (22) each with 64 Hosts (26) will work
c) .nnnhhhhh gives 8 Subnets (23) each with 32 Hosts (25) not enough Hosts
Example b is our choice. Here
are the 4 possible bit patterns for the Subnet: 00, 01, 10, 11.
Now, let's consider the Host bit patterns. The first available
pattern for each network is 000001 (remember, can't be all 0's)
and the highest bit pattern is 111110 (remember, can't be all
1's). Now, let's show all 8 bits:
Network # first Host # last Host #
network 1: 00000000 (0 decimal) 00000001 (1 decimal) 00111110 (62 decimal)
network 2: 01000000 (64 decimal) 01000001 (65 decimal) 01111110 (126 decimal)
network 3: 10000000 (128 decimal) 10000001 (129 decimal) 10111110 (191 decimal)
network 4: 11000000 (192 decimal)
11000001 (193 decimal) 11111110 (254 decimal)
Notice that when we converted
the byte to decimal, we used all 8 bits, even though some bits
were used for Subnets and other bits were used for Hosts. Never
split the 8 bit byte when converting to decimal.
Now, let's show all 4 bytes in decimal:
Network IP# first Host IP# last Host IP#
network 1: 206.71.95.0 206.71.95.1 206.71.95.62
network 2: 206.71.95.64 206.71.95.65 206.71.95.126
network 3: 206.71.95.128 206.71.95.129 206.71.95.191
network 4: 206.71.95.192 206.71.95.193
206.71.95.254
There's one last requirement
of Subnetting, called the Subnet Mask number. It will be explained
a little later. All you need to know for now is that it's used
to recognize the Subnets that you've created. With the Mask,
all devices on your internet will be able to distinguish that
the Host 206.71.95.68 is on Network 206.71.95.64 while the Host
206.71.95.130 is on Network 206.71.95.128, despite the fact that
they both share the same Primary Network numbers of 206.71.95.
Two More Rules
a) Subnets of zero are prohibited,
so the first Network 206.71.95.0 and all associated Host addresses
(206.71.95.1 through 206.71.95.62) should not be used. These
are called the all 0's Subnets.
b) Subnets of all 1's are strongly discouraged. Avoid
using the value of 255 for any Subnet byte. For example, if the
Primary Network address is 167.100.0.0, it is preferable to avoid
the Subnet 167.100.255.0.
Let's look at Subnetting a class B Network Address because it presents some interesting combinations not available with a class C address. We'll use 195.153.0.0. We now have 2 bytes (16 bits) to play with.
11000011.10011001.00000000.00000000
Again, let's ignore the first 2 bytes since they can't be changed. There are several possibilities for Subnetting. For example, you could use all 8 bits of the 3rd byte for Subnetting (called even byte boundary Subnetting) which would give you all bits in the 4th byte for Host addressing. Or, you could split them up any way you want. Some other examples:
nnnnhhhh.hhhhhhhh, or nnnnnnnn.nnhhhhhh, and so on.
Let's look at even byte-boundary Subnetting (using the entire 3rd byte). This gives you 254 Subnets ranging from 00000001.00000000 (1.0 decimal) to 11111110.00000000 (254.0 decimal). Remember, we're avoiding the all 0's and all 1's Subnets. Hence, the resulting Subnet addresses ranges from 195.153.1.0 to 195.153.254.0. The host addressing is done in the same way as the first example, but using all 8 bits of the 4th byte.
Here's another possibility using the entire 3rd byte PLUS 2 bits of the 4th byte. This is called bit-level Subnetting. This gives you 1022 Subnets ranging from 00000000.01000000 (0.64 decimal) to 11111111.10000000 (255.192 decimal). Again, we're not using the all 0's and all 1's Subnets. Hence the resulting Subnet addresses ranges from 195.153.0.64 to 195.153.255.192. Again, Host addressing is done the same way, but using only 6 bits of the 4th byte. Remember, with bit-level Subnetting, when converting your bits to decimal, use all 8 bits of each byte and don't split bytes.
As you can see, your Subnetting
options are very flexible if you just follow a couple of simple
rules. Think about what your organization's needs are before
requesting a particular class of Network Address, and before
deciding how to Subnet.
The Subnet Mask, Also Known As The Mask
One final rule of Subnetting involves the Subnet Mask. Now that you've divided your Primary Network address in to Subnets, you need a way to tell the bridges, routers and hosts on your internet about your clever Subnet scheme. Without a way to do this, there's no way to tell if a particular Host address belongs to a Subnet or the Primary Network. Look at the following address:
193.192.32.160
Is this host # 160 on Network 193.192.32.0... is it host # 32.160 on Network 193.192.0.0...is it host # 32 on Network 193.192.32.128... or is it a Subnet number?
They're all possibilities, but you can't tell unless I tell you what the Primary Network address is and which Subnet scheme I used, i.e., which bits were used for Subnetting and which bits were used for Host addressing.
The answer is the Subnet Mask,
which is a specially constructed number that tells you (and the
computers, bridges & routers) which bits were used for Subnetting.
Here are the rules for constructing the Subnet Mask:
a) The Mask consists of 4 bytes, using the Primary Network address bits, Subnet bits and Host bits to determine its final form.
b) All bits used for the Primary Network address get turned into 1's
c) All bits used for Subnetting also get turned into 1's.
d) All bits used for Host addressing get turned into 0's.
e) The resulting four 8-bit bytes get converted to a decimal value.
Now, let's apply these rules
to an example. Let's choose any class C Network address like
193.192.32.0.
First, we need to decide which
bits will be used for Subnetting. Remembering our earlier rules,
we see that the first 3 bytes (193.192.32) cannot be altered;
however, all of the bits in the 4th byte are available for Subnetting
and Host addressing. Let's arbitrarily choose the first 3 bits
of the 4th byte to be used for Subnetting. Here's our results
after applying the rules:
11111111.11111111.11111111.11100000
Now, converting to decimal, we get: 255.255.255.224
There you have it, the Subnet Mask for a class C Network address using 3 bits for Subnetting. After you tell your computers, routers and bridges this value, they'll recognize all of your Subnets and treat them as different physical networks.
Let's look at all of the Subnet value possibilities for this class of address. Ignoring the first 3 bytes since they can't be changed, we get:
00000000 (prohibited because it's all 0's)
10000000 128 (255.255.255.128)
11000000 192 (255.255.255.192)
11100000 224 (255.255.255.224) our example
11110000 240 (255.255.255.240) limited, only allows 14 Hosts (1111 and 0000 not allowed)
11111000 248 (255.255.255.248) very limited, only allows 6 Hosts (111 and 000 not allowed)
11111100 252 (255.255.255.252) dumb, only allows 2 Hosts (11 and 00 not allowed)
11111110 254 (255.255.255.254) very dumb, no Hosts available (1 and 0 not allowed)
11111111 255 (255.255.255.255) very dumb, no Hosts available (black holes not allowed)
These Mask values of 128, 192, 224 and so on are the only ones possible within a single byte. Learn to recognize them and their associated number of Subnets and Hosts.
Just a brief example of some
class B Subnet Masks:
10000000.00000000 128.0 (255.255.128.0)
11110000.00000000 240.0 (255.255.240.0)
11111110.00000000 254.0 (255.255.254.0)
11111111.00000000 255.0 (255.255.255.0) not recommended, but not prohibited
11111111.11100000 255.224 (255.255.255.224)
not recommended, but not prohibited
See how the byte values of 128,
240, 254, etc. have appeared again, this time in both the 3rd
and 4th bytes.
6. GATEWAYS
In section 2 "Networks and Hosts", the terms "physical network" and "directly connected" were mentioned. Briefly, these terms indicate the existence of a physical boundary. Inside this physical boundary lies an individual network with all its computers, network adapters, printers and such. Outside this boundary exists other networks which have their own physical boundaries. Also mentioned in section 2 is the fact that each individual network within a physical boundary has its own unique IP network address.
Components residing inside different boundaries (on different physical networks with different network addresses) are not considered to be directly connected, and yet they are able to communicate with each other. Obviously, they are connected in some manner. This connection is typically accomplished by a bridge or router.
The illustration from section
2 is reproduced below. Note that for this example, within each
bridge there must be 2 physical network connections (or ports),
one for each of the two networks that are bridged. For example,
Bridge-A is bridging network 1 to network 2, therefore Bridge-A
has both a network 1 port and a network 2 port. These ports,
just like any other host, each have an individual IP address.
As previously discussed, a host IP address is a sub-set of the
network address on which it resides. Arbitrary network and host
addresses have been added to the illustration.
OK, so now we have a bridge between network 1 and network 2. However, a host on network 1 won't be able to talk to a host on network 2 until we do a very special thing... tell the hosts the path by which to reach each other. This is known as the GATEWAY.
The Gateway for a particular host is any
bridge port that's on the same physical network as the host.
In our example above, the Gateway for all hosts on network 1
is 206.71.95.27. The Gateway for all hosts on network 2 is 185.22.98.143.
Network 1 and 2 hosts that need to talk to each other must both
have their appropriate Gateways defined in whatever TCP/IP software
they're using.
What if a host on network 1 needs
to talk to a host on network 3? It is NOT enough to simply have
gateways defined for the hosts on network 1, 2 and 3. In this
case, the bridge ports need to have gateways defined.
The Gateway for a particular bridge port is any
other bridge port that's on the same physical network as the
particular bridge port. For example, the gateway for bridge port
185.22.98.143 is bridge port 185.22.98.141... and vice versa
if 2 way communication is to happen. Let me explain the vice
versa thing. If bridge port 185.22.98.143 is the only one with
a defined gateway, then network 3 hosts will not be able to send
data to network 1 hosts although network 1 hosts will be able
to send data to network 3 hosts. This is not necessarily a bad
thing, in fact it can be one way to "firewall" or prevent
unwanted hacking from outside sources.
Notice that there's a common rule to Gateways. A Gateway for
any given device must reside on the same physical network as
the device, i.e., the device and it's gateway are "directly
connected".
