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	255.255.255.0	0	254
/25	255.255.255.128	2 (0)	126
/26	255.255.255.192	4 (2)	62
/27	255.255.255.224	8 (6)	30
/28	255.255.255.240	16 (14)	14
/29	255.255.255.248	32 (30)	6
/30	255.255.255.252	64 (62)	2