User's Manual

Page 902 of 910 Appendix E: Subnet Mask Conversion from CIDR to Dotted Decimal Format
553-3001-365 Standard 4.00 August 2005
Procedure 129
Converting a subnet mask from CIDR format to dotted decimal format
1 Divide the CIDR format value by 8. The quotient (the number of times that
eight divides into the CIDR format value) equals the number of dotted
decimal fields containing 255.
In the example above, the subnet mask is expressed as /20. Twenty
divided by eight equals a quotient of two, with a remainder of four.
Therefore, the first two fields of the subnet mask in dotted decimal format
are 255.255.
2 If there is a remainder, refer to Table 123 to obtain the dotted decimal
value for the field following the last field containing “255”. In the example
of /20 above, the remainder is four. In Table 123, a remainder of four
equals a binary value of 1111 0000 and the dotted decimal value of the
next and last field is 240. Therefore the first three fields of the subnet
mask are 255.255.240.
3 If there are any remaining fields in the dotted decimal format, they have a
value of 0. Therefore, the complete subnet mask in dotted decimal format
is 255.255.240.0.
End of Procedure
Table 123
CIDR format remainders
Remainder of CIDR
format value divided
by eight
Binary value Dotted decimal value
1
1000 0000 128
2 1100 0000 192
3 1110 0000 224
4 1111 0000 240
5 1111 1000 248
6 1111 1100 252
7
1111 1110 254