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Appendix D
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.
Follow the steps in Procedure 76 "Converting a subnet mask from CIDR
format to dotted decimal format" (page 455) to convert a subnet mask from
CIDR format to dotted decimal format.
Procedure 76
Converting a subnet mask from CIDR format to dotted decimal format
Step Action
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 87 "CIDR format remainders"
(page 456) to get the dotted decimal value for the field following the
last field containing "255".
Nortel Communication Server 1000
IP Trunk Fundamentals
NN43001-563 01.01 Standard
Release 5.0 30 May 2007
Copyright © 2007, Nortel Networks
.