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Originally Posted by jtwn
The 'cust' part now doesn't appear to relate to your address anymore.
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Yes the 'cust' part does still relate to your IP address, but not in the obvious way that previously applied. Previously, each customer-side interface to the CMTS (or uBR), as identified by card and cable number, had its own set of IP subnet ranges, each usually quite small, for instance with a netmask of 255.255.255.0. In that system, the custXXX element of the DNS name was identical to the final octet of the IP address.
In the new system, many interfaces on a single CMTS have been bundled together for the purposes of IP address allocation, resulting in fewer and larger IP subnets spanning several CMTS interfaces. A strong motivation to do this is to economise on the number of IPv4 addresses wasted and unusable when many small subnets are used. Better utilisation of the increasingly scarce IPv4 addresses can be achieved with fewer larger subnets.
For instance, my connection to Cambridge UBR 8 now has a netmask of 255.255.252.0, a subnet of size 1024 instead of the previous size of 256. The gateway address has become 82.6.104.1 (instead of being a .254 address). User IP addresses are available from 82.6.104.2 (DNS cust1) through to 82.6.107.254 (DNS cust1021). The correspondence between IP address and the DNS custXXX element is still there, if you do the sums.