Netblocks explained

By : Christopher Wray | August 08, 2007 | Blog

Netblocks explained

August 08, 2007

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.

Class C
Network Bits Subnet Mask Number of Subnets Number of Hosts
/24 0 254
/25 2 (0) 126
/26 4 (2) 62
/27 8 (6) 30
/28 16 (14) 14
/29 32 (30) 6
/30 64 (62) 2

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