Today we had a client query how many IP Address’s they had allocated to their Colocation cabinet. We explained that they had a /29 allocation, with 8 IP Addresses. The confusion came in when we told them only 5 were usable. For an explanation on how IPv4 Subnetting works, and how IP Address space is allocated check out this useful table explaining IPV4 Networks.
So why only 5 usable addresses on a /29 allocation when 8 are available?
This is because 1 is used for Broadcast, 1 used for Network and another is used as your Gateway IP address.
Below is an explanation of a Class C allocation, showing “Number of Hosts” as the theoretical number of usable IP addresses, however you always need to minus 1 for your Gateway IP address on our network.
|Network Bits||Subnet Mask||Number of Subnets||Number of Hosts|