Details about Internet addresses: subnets and broadcasting
As indicated above, Internet addresses are 32-bit numbers, normally
written as 4 octets (in decimal), e.g. 220.127.116.11. There are actually
3 different types of address. The problem is that the address has to
indicate both the network and the host within the network. It was
felt that eventually there would be lots of networks. Many of them
would be small, but probably 24 bits would be needed to represent all
the IP networks. It was also felt that some very big networks might
need 24 bits to represent all of their hosts. This would seem to lead
to 48 bit addresses. But the designers really wanted to use 32 bit
addresses. So they adopted a kludge. The assumption is that most of
the networks will be small. So they set up three different ranges of
address. Addresses beginning with 1 to 126 use only the first octet
for the network number. The other three octets are available for the
host number. Thus 24 bits are available for hosts. These numbers are
used for large networks. But there can only be 126 of these very big
networks. The Arpanet is one, and there are a few large commercial
networks. But few normal organizations get one of these class A
addresses. For normal large organizations, class B addresses are
used. Class B addresses use the first two octets for the network
number. Thus network numbers are 128.1 through 191.254. (We avoid 0
and 255, for reasons that we see below. We also avoid addresses
beginning with 127, because that is used by some systems for special
purposes.) The last two octets are available for host addesses,
giving 16 bits of host address. This allows for 64516 computers,
which should be enough for most organizations. (It is possible to get
more than one class B address, if you run out.) Finally, class C
addresses use three octets, in the range 192.1.1 to 223.254.254.
These allow only 254 hosts on each network, but there can be lots of
these networks. Addresses above 223 are reserved for future use, as
class D and E (which are currently not defined).
Many large organizations find it convenient to divide their network
number into subnet. For example, Rutgers has been assigned a class
B address, 128.6. We find it convenient to use the third octet of the
address to indicate which Ethernet a host is on. This division has no
significance outside of Rutgers. A computer at another institution
would send any packet whose destination address began with 128.6 on
the best route to Rutgers. They would not have different routes for
128.6.4 or 128.6.5. But inside Rutgers, we treat 128.6.4 and 128.6.5
as separate networks. In effect, gateways inside Rutgers have
separate entries for each Rutgers subnet, whereas gateways outside
Rutgers just have one entry for 128.6. Note that we could do exactly
the same thing by using a separate class C address for each Ethernet.
As far as Rutgers is concerned, it would be just as convenient for us
to have a number of class C addresses. However using class C
addresses would make things inconvenient for the rest of the world.
Every institution that wanted to talk to us would have to have a
separate entry for each one of our networks. If every institution did
this, there would be far too many networks for any reasonable gateway
to keep track of. By subdividing a class B network, we hide our
internal structure from everyone else, and save them trouble. This
subnet strategy requires special provisions in the network software.
It is described in rfc950.txt.
0 and 255 have special meanings. 0 is reserved for machines that
don't know their address. In certain circumstances it is possible for
a machine not to know the number of the network it is on, or even its
own host address. So 0.0.0.23 would be a machine that knew it was
host number 23, but didn't know on what network.
255 is used for broadcast. A broadcast is a message that you want
every system on the network to see. Broadcasts are used in some
situations where you don't know who to talk to. For example, suppose
you need to look up a host name and get its Internet address.
Sometimes you don't know the address of the system that has the host
name data base. In that case, you might send the request as a
broadcast. There are also cases where a number of systems are
interested in information. It is then less expensive to send a single
broadcast than to send packets individually to each host that is
interested in the information. In order to send a broadcast, you use
an address that is made by using your network address, with all ones
in the part of the address where the host number goes. For example,
if you are on network 128.6.4, you would use 18.104.22.168 for
broadcasts. How this is actually implemented depends upon the medium.
It is not possible to send broadcasts on the Arpanet, or on point to
point lines. However it is possible on an Ethernet. If you use an
Ethernet address with all its bits on (all ones), every machine on the
Ethernet is supposed to look at that packet.
Although the official broadcast address for network 128.6.4 is now
22.214.171.124, there are some other addresses that may be treated as
broadcasts by certain implementations. For convenience, the standard
also allows 255.255.255.255 to be used. This refers to all hosts on
the local network. It is often simpler to use 255.255.255.255 instead
of finding out the network number for the local network and forming a
broadcast address such as 126.96.36.199. In addition, certain older
implementations may use 0 instead of 255 to form the broadcast
address, e.g. 188.8.131.52. Finally, certain older implementations may
not understand about subnets. Thus they consider the network number
to be 128.6. In that case, they will assume a broadcast address of
184.108.40.206 or 220.127.116.11. Until support for broadcasts is
implemented properly, it can be a somewhat dangerous feature to use.
Because 0 and 255 are used for unknown and broadcast addresses, normal
hosts should never be given addresses containing 0 or 255. Addresses
should never begin with 0, 127, or any number above 223. Addresses
violating these rules are sometimes referred to as Martians, because
of rumors that the Central University of Mars is using network 225.