192

If the subnet mask for an octet is represented by 255, all the equivalent bits in that octet of the address are network bits. Similarly, if the subnet mask for an octet is represented by 0, all the equivalent bits in that octet of the address are host bits. In each of these cases, it is not necessary to expand this octet to binary to determine the network and host portions.

Similarly, you can use the other patterns to determine the number of network and data bits inside the octet.

IPv4 Experimental Address Range

Expressed in dotted decimal format, the IPv4 address range is 0.0.0.0 to 255.255.255.255. As you have already seen, not all these addresses can be used as unicast host addresses for unicast communication. Recall that the block of addresses from 224.0.0.0 to 239.255.255.255 is reserved for the addressing of multicast groups.

All the IPv4 addresses higher than the multicast range are also reserved for special purpos-es. Except for the limited broadcast address of 255.255.255.255, the address range of

240.0.0.0 to 255.255.255.254 is the IPv4 experimental addresses. Currently, these addresses are listed as reserved for future use (RFC 3330). This suggests that they could be converted to usable addresses. Currently, they cannot be used in IPv4 networks. However, these addresses are used for research or experimentation.

After accounting for the ranges reserved for experimental addresses and multicast address-es, this leaves an address range of 0.0.0.0 to 223.255.255.255 that could be used for IPv4 hosts. However, this range includes many addresses that are already reserved for special purposes. Although some of these addresses have been previously presented, the major reserved addresses are discussed in the next sections.

Public and Private Addresses

Although most IPv4 host addresses are public addresses designated for use in networks that are accessible on the Internet, there are blocks of addresses used in networks that require limited or no Internet access. These addresses are called private addresses.

The private address blocks are

■   10.0.0.0 /8 (10.0.0.0 to 10.255.255.255)

■   172.16.0.0 /12 (172.16.0.0 to 172.31.255.255)

■   192.168.0.0 /16 (192.168.0.0 to 192.168.255.255)

Private space address blocks are set aside for use in private networks. The use of these addresses need not be unique among outside networks. Hosts that do not require access to the Internet at large can make unrestricted use of private addresses. However, the internal networks still must design network address schemes to ensure that the hosts in the private networks use IP addresses that are unique within their networking environment.