Chapter 3: Installation
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Class C:
The IP address ranges between 192.0.0.0 and 223.255.255.255. Each class C network has a 24-bit network prefix followed by an 8-bit host
address. A total of 2,097,152 (221)/24 networks can be defined with a maximum of 254(28-2) hosts per network.
Class D and E:
Class D is a class with the first 4 MSBs (Most Significant Bits) set to 1-1-1-0 and is used for IP Multicast. See also RFC 1112. Class E is a
class with the first 4 MSBs set to 1-1-1-1 and is used for IP broadcast.
According to IANA (Internet Assigned Numbers Authority), three specific IP address blocks (called a private IP address) are reserved for
extending an internal network. They are listed below.
Class A 10.0.0.0---10.255.255.255
Class B 172.16.0.0---172.31.255.255
Class C 192.168.0.0---192.168.255.255
Refer to RFC 1597 and RFC 1466 for more information. These documents are available at
www.faqs.org
.
3.5.2 Subnet Mask
Subnet mask is the sub-division of a class-based network or a CIDR block. The subnet is used to determine how to split an IP address into
the network prefix and the host address. It’s designed to use an IP address more efficiently to manage an IP network.
For a class B network, 128.1.2.3, the default subnet mask may be 255.255.0.0. The first two bytes are all 1s. This means more than 60
thousands of nodes in flat IP addresses will be on the same network. It’s too large to manage practically. If we divide it into smaller
networks by extending the network prefix from 16 bits to, say 24 bits, the network uses its third byte to subnet this class B network. The
subnet mask is 255.255.255.0; each bit of the first three bytes is 1. The first two bytes are used to identify the class B network, the third
byte is used to identify the subnet within this class B network, and the last byte is the host number.
Not all IP addresses are available in the subnetted network. Two special addresses are reserved. They are the addresses with all zeros and
all ones for the host number.
As shown in the table below, the subnet mask with a 25-bit long, 255.255.255.128 address contains 126 members in the subnetted
network. The network prefix length equals the bit number with 1s in that subnet mask. Use this table to count the number of IP addresses
matched.
Table 3-2. Subnet mask values.
Prefix Length
Number of IPs Matched
Number of Addressable IPs
/32 1 -
/31 2 -
/30 4 2
/29 8 6
/28 16 14
/27 32 30
/26 64 62
/25 128 126
/24 256 254
/23 512 510
/22 1024 1022
/21 2048 2046
/20 4096 4094
/19 8192 8190
/18 16384 16382
/17 32768 32766
/16 65536 65534