IP4 netmask/subnet address reference table. Includes hexadecimal, decimal and binary tables.

Netmask Table

Prefix Dotted Decimal     Hexadecimal   Binary Netmask
Length Netmask            Netmask
----------------------------------------------------------------------------
/0     0.0.0.0            0x00000000    00000000 00000000 00000000 00000000
/1     128.0.0.0          0x80000000    10000000 00000000 00000000 00000000
/2     192.0.0.0          0xc0000000    11000000 00000000 00000000 00000000
/3     224.0.0.0          0xe0000000    11100000 00000000 00000000 00000000
/4     240.0.0.0          0xf0000000    11110000 00000000 00000000 00000000
/5     248.0.0.0          0xf8000000    11111000 00000000 00000000 00000000
/6     252.0.0.0          0xfc000000    11111100 00000000 00000000 00000000
/7     254.0.0.0          0xfe000000    11111110 00000000 00000000 00000000
/8     255.0.0.0          0xff000000    11111111 00000000 00000000 00000000

/9     255.128.0.0        0xff800000    11111111 10000000 00000000 00000000
/10    255.192.0.0        0xffc00000    11111111 11000000 00000000 00000000
/11    255.224.0.0        0xffe00000    11111111 11100000 00000000 00000000
/12    255.240.0.0        0xfff00000    11111111 11110000 00000000 00000000
/13    255.248.0.0        0xfff80000    11111111 11111000 00000000 00000000
/14    255.252.0.0        0xfffc0000    11111111 11111100 00000000 00000000
/15    255.254.0.0        0xfffe0000    11111111 11111110 00000000 00000000
/16    255.255.0.0        0xffff0000    11111111 11111111 00000000 00000000

/17    255.255.128.0      0xffff8000    11111111 11111111 10000000 00000000
/18    255.255.192.0      0xffffc000    11111111 11111111 11000000 00000000
/19    255.255.224.0      0xffffe000    11111111 11111111 11100000 00000000
/20    255.255.240.0      0xfffff000    11111111 11111111 11110000 00000000
/21    255.255.248.0      0xfffff800    11111111 11111111 11111000 00000000
/22    255.255.252.0      0xfffffc00    11111111 11111111 11111100 00000000
/23    255.255.254.0      0xfffffe00    11111111 11111111 11111110 00000000
/24    255.255.255.0      0xffffff00    11111111 11111111 11111111 00000000

/25    255.255.255.128    0xffffff80    11111111 11111111 11111111 10000000
/26    255.255.255.192    0xffffffc0    11111111 11111111 11111111 11000000
/27    255.255.255.224    0xffffffe0    11111111 11111111 11111111 11100000
/28    255.255.255.240    0xfffffff0    11111111 11111111 11111111 11110000
/29    255.255.255.248    0xfffffff8    11111111 11111111 11111111 11111000
/30    255.255.255.252    0xfffffffc    11111111 11111111 11111111 11111100
/31    255.255.255.254    0xfffffffe    11111111 11111111 11111111 11111110
/32    255.255.255.255    0xffffffff    11111111 11111111 11111111 11111111

Source: http://www.bgp4.as/