netmasks ��

Angsuman Chakraborty ��

October 16th, 2007 �� / �� 16th � 2007

Network masks is a notation to group IP addresses. �� . Let's take a familiar analogy. �� . My phone in US was 510-595-3830. �� 510-595-3830. 510 was the area code which included a large group of phone numbers in San Francisco-Oakland-Emeryville area. 510 �� - �� - Emeryville ��. Similarly IP addresses are also grouped by their network prefix. �� .

Netmasks are important for allocating sub-networks to individuals or organizations. Netmasks �� . They are also used to identify networks for routing purposes. �� .

Each (IPv4) IP address contains a 4 8-bit integers in the form abcd So an IPv4 IP address contains 32 bits. �� (ipv4) �� 4 8 �� ABCD �� ipv4 �� 32 ��. A netmask indicates the number of bits which are fixed for all IP addresses within the network. � netmask �� .

Let's take a network which contains IP addresses between 72.31.32.0 - 72.31.32.255. �� 72.31.32.0 -- 72.31.32.255. In this network the first 24 bits (first 3 8-bit integers) are constant for all IP addresses. �� 24 �� (�� 3 8 �� ) �� . So the network mask for this network will be denoted as /24 . �� / 24.

There is a longer notation to describe network masks which is used in Windows and also Linux to specify the netmask of your own computer. �� netmask �� . Here instead of writing the number of constant bits in the IP address in decimal format you need to specify it in the IP address format. �� . So /24 becomes 255.255.255.0. �� / 24 �� 255.255.255.0. The first three 255 represents the 24 bits of the address. �� 255 �� 24 �� .

Each network specified by the netmask can contain a maximum number of computers or IP address bearing devices (like routers) as limited by the netmask. �� netmask �� (�� ) �� netmask. For example a /24 network can contain a maximum of 2 ⁸ -1 (255) computers. �� / 24 �� 2 ⁸ -1 (255) ��. One of the address (highest one) is reserved for broadcast address of the network. �� (�� ) �� . The number 8 is derived by subtracting 24 from 32. �� 8 �� 24 �� 32.

Refer to the table below for network masks in both forms and maximum number of machines for quick reference: �� :

Short Form ��	Full Form ��	Maximum number of machines ��	Comment ��
/8 / 8	/255.0.0.0 / 255.0.0.0	16, 777, 215 16 � 777 � 215	Class A address ��
/16 / 16	/255.255.0.0 / 255.255.0.0	65, 535 65 � 535	Class B address ��
/17 / 17	/255.255.128.0 / 255.255.128.0	32, 767 32 � 767
/18 / 18	/255.255.192.0 / 255.255.192.0	16, 383 16 � 383
/19 / 19	/255.255.224.0 / 255.255.224.0	8, 191 8 � 191
/20 / 20	/255.255.240.0 / 255.255.240.0	4, 095 4 � 095
/21 / 21	/255.255.248.0 / 255.255.248.0	2, 047 2 � 047
/22 / 22	/255.255.252.0 / 255.255.252.0	1, 023 1 � 023
/23 / 23	/255.255.254.0 / 255.255.254.0	511
/24 / 24	/255.255.255.0 / 255.255.255.0	255	Class C address � ��
/25 / 25	/255.255.255.128 / 255.255.255.128	127
/26 / 26	/255.255.255.192 / 255.255.255.192	63
/27 / 27	/255.255.255.224 / 255.255.255.224	31
/28 / 28	/255.255.255.240 / 255.255.255.240	15
/29 / 29	/255.255.255.248 / 255.255.255.248	7
/30 / 30	/255.255.255.252 / 255.255.255.252	3