96
Appendix 6 – Addresses, masks and ports
IP address, network masks and ports are all closely linked in the quest for one
device to find another across disparate network links.
IP addresses
As a rough analogy, consider how you use the telephone system. The phone
number for LINDY in the UK is
0044 (0)1642 754000
. This number consists of
three distinct parts:
•
0044
connects from another country to the UK
•
(0)1642
selects the main telephone exchange in the Thornaby area of
Stockton-on-Tees, and
•
754000
is the unique code for LINDY within Thornaby.
The important parts of the whole number depend on where you are. If you were
based in the same local area as LINDY, there would be no point in dialling out of
the UK, or even out of the area. The only part of the whole number that you are
interested in is the final part: 754000.
In a similar way to the various parts of the telephone number, the four sections
(or
Octets
) of every IP address have different meanings or “weights”. Consider
the following typical IP address:
192.168.142.154
192
is the most global part of the number (akin to the
0044
of the phone
number) and
154
is the most local (similar to the
754000
unique local code of
the phone number).
When two network devices communicate with each other, they always “dial the
whole number” regardless of their respective locations in a network. However,
they still need to know whether the other device is local to them or not, and this
is where the net mask comes into play.
Net masks
The net mask (or sub-net mask) informs a device as to its own position within a
network. From this it can determine whether any other device is within the same
local network or is situated further afield.
Taking the telephone number analogy given in the IP address section, in order
to use the telephone system efficiently, it is vital for you to know your location
relative to the person you are calling. In this way you avoid dialling unnecessary
numbers.
When one network device needs to talk to another, the first thing that it will
do is a quick calculation using its own IP address, the other device’s IP address
and its own net mask. Suppose a device with address
192.168.142.154
and
net mask
255.255.255.0
needed to communicate with a device at address
192.168.142.22
. The sending device would perform several calculations:
The reason for doing this? It makes the network, as a whole, much more
efficient. If every message for every recipient was shoved straight out onto the
Internet, the whole thing would grind to a halt within seconds. Net masks keep
local traffic just that - local.
Want to know more?
1
The net mask is used to determine the local and global parts of the sender’s IP
address. Where there is 255 in the mask, the corresponding address slips through,
where there is a 0, it is blocked.
2
Where the net mask was 0, the corresponding part of the result is also zero - this
section is now known to be the local part of the IP address.
3
The same process is carried out for the destination address, again using the sender’s
net mask. Now the local parts of both addresses have been equalised to zero, because
their values are not important in determining whether they are both in the same local
network.
4
The results of the two net mask operations are now compared, if they match, the
destination is local. If not, then the sender will still use the same full destination IP
address but will also flag the message to go via the local network gateway and out
into the wider world.
Sending
device IP
address
Net mask
Destination
device IP
address
192.168.142.000
=
192.168.142.000
? Answer:
YES
Address
is local
1
2
4
192
192
168
168
142
142
154
000
192
192
168
168
142
142
22
000
Result
3
