Netmask

Easy explanation of netmask.

Visual reference:

    Class   Address # of Hosts  Netmask (Binary)    Netmask (Decimal)
    CIDR    /4      240,435,456 11110000 00000000 00000000 00000000 240.0.0.0
    CIDR    /5      134,217,728 11111000 00000000 00000000 00000000 248.0.0.0
    CIDR    /6       67,108,864 11111100 00000000 00000000 00000000 252.0.0.0
    CIDR    /7       33,554,432 11111110 00000000 00000000 00000000 254.0.0.0
    A   /8           16,777,216 11111111 00000000 00000000 00000000 255.0.0.0
    CIDR    /9        8,388,608 11111111 10000000 00000000 00000000 255.128.0.0
    CIDR    /10       4,194,304 11111111 11000000 00000000 00000000 255.192.0.0
    CIDR    /11       2,097,152 11111111 11100000 00000000 00000000 255.224.0.0
    CIDR    /12       1,048,576 11111111 11110000 00000000 00000000 255.240.0.0
    CIDR    /13         524,288 11111111 11111000 00000000 00000000 255.248.0.0
    CIDR    /14         262,144 11111111 11111100 00000000 00000000 255.252.0.0
    CIDR    /15         131,072 11111111 11111110 00000000 00000000 255.254.0.0
    B   /16              65,534 11111111 11111111 00000000 00000000 255.255.0.0
    CIDR    /17          32,768 11111111 11111111 10000000 00000000 255.255.128.0
    CIDR    /18          16,384 11111111 11111111 11000000 00000000 255.255.192.0
    CIDR    /19           8,192 11111111 11111111 11100000 00000000 255.255.224.0
    CIDR    /20           4,096 11111111 11111111 11110000 00000000 255.255.240.0
    CIDR    /21           2,048 11111111 11111111 11111000 00000000 255.255.248.0
    CIDR    /22           1,024 11111111 11111111 11111100 00000000 255.255.252.0
    CIDR    /23             512 11111111 11111111 11111110 00000000 255.255.254.0
    C   /24                 256 11111111 11111111 11111111 00000000 255.255.255.0
    CIDR    /25             128 11111111 11111111 11111111 10000000 255.255.255.128
    CIDR    /26              64 11111111 11111111 11111111 11000000 255.255.255.192
    CIDR    /27              32 11111111 11111111 11111111 11100000 255.255.255.224
    CIDR    /28              16 11111111 11111111 11111111 11110000 255.255.255.240
    CIDR    /29               8 11111111 11111111 11111111 11111000 255.255.255.248
    CIDR    /30               4 11111111 11111111 11111111 11111100 255.255.255.252

Netmask /32 applies to a single IP address.