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Page 357 of
378
ITG Trunk 2.0 ISDN Signaling Link (ISL) Description, Installation and Operation
358
Appendix C: Subnet mask conversion
from CIDR to dotted
decimal format
Subnet masks can be expressed in Classless Inter Domain Routing (CIDR)
format, appended to the IP address (for example, 10.1.1.1/20). The subnet
mask must be converted from CIDR format to dotted decimal format to
configure ITG IP addresses.
CIDR format expresses the subnet mask as the number of bits counting from
the most significant bit of the first IP address field. A complete IP address
consists of 32 bits. A typical CIDR format subnet mask is in the range from
/9 to /30. Each decimal number field in the dotted decimal format can have a
value from 0 to 255, where 255 represents binary 1111 1111.
To convert a subnet mask from CIDR format to dotted decimal format:
1 Divide the CIDR format value by 8. The result is equal to the number
of dotted decimal fields containing 255.
In the example above, (10.1.1.1/20), the subnet mask is /20. 20 divided
by 8 is equal to 2, with a remainder of 4. The first 2 fields of the subnet
mask in dotted decimal format are 255.255.
2 If there is a remainder, refer to Table 70, “CIDR format remainders,” on
page 358 to get the dotted decimal value for the field following the last
field containing “255”.
In the example of /20 above, the remainder is 4. In Table 70, “CIDR
format remainders,” on page 358, a remainder of 4 is equal to a binary
value of 1111 0000 and the dotted decimal format value of the next and
last field is 240. The first 3 fields of the subnet mask are 255.255.240.