TCP/IP Configuration and Management Manual

NonStop TCP/IP Processes and Protocols
TCP/IP Configuration and Management Manual—427132-004
B-4
The Problem of Resolving Addresses
networks can exist; however, each Class A network can have as many as 16,777,214
hosts, with addresses from %H000001 through %HFFFFFE (local addresses
%H000000 and %HFFFFFF are reserved).
A Class B address consists of a two-octet network address and a two-octet local
address. The two high-order bits of the network address are always 10; therefore, the
first octet is a number in the range 128 through 191 (%H80 through %HBF). So,
16,384 Class B networks can exist, each with as many as 65,534 hosts, with host
addresses from %H0001 through %HFFFE (local addresses %H0000 and %HFFFF
are reserved).
A Class C address consists of a three-octet network address and a one-octet local
address. The three high-order bits of the network address are always 110; therefore,
the first octet is a number in the range 192 through 223 (%HC0 through %HDF). So,
2,097,152 Class C networks can exist, each with 254 hosts, with host addresses from
%H01 through %HFE (local addresses %H00 and %HFF are reserved).
A Class D address is a 4-octet multicast group address. The four high-order bits of the
address are always 1110; therefore, the first octet is a number in the range 224 through
239 (%HE0 through %HEF). This means that an Internet can have a total of
268,435,456 multicast groups.
Class E addresses (240.0.0.0 to 247.255.255.255) are not currently supported and are
treated as Class C addresses.
Broadcasting Using IP Addresses
As described above, a local address (hostid) consisting of all 1’s is reserved for
broadcast messages. The Internet protocols normally restrict the broadcast to the
network on which the broadcast message originates. The relative efficiency of
broadcast messages depends on the network technology employed. Some networks
do not support broadcast messages, while others (such as Ethernet) handle them just
like any other transmission. You should be aware of the configuration of your network
and how it handles broadcast messages before you attempt to use them.
The Problem of Resolving Addresses
In the previous subsection, we discussed the concepts of Internet addressing. In
practice, these concepts can be very complex to implement. One of the complexities of
implementation involves the mapping of Internet (IP) addresses to hardware
addresses. The mapping of IP addresses to hardware addresses and how that is
achieved is the topic of this subsection.
Note. The local address 0 is never assigned to an individual host. Internet addresses that
have a local address of 0 are always refer to the network itself. The local address of all 1’s is
also never assigned to an individual host. Internet addresses that have a local address of all
1’s are always reserved for broadcast.