Network Subnets (CIDR) and Usable IP Addresses Reference Guide

When commissioning internet connections, one important aspect is calculating how many public IP addresses (IPs) are needed to fulfil all of the business IT requirements.

IPs are deployed as part of a subnet, or as your ISP may call it informally, an “IP block”.

To specify the subnet, a CIDR (Classless Inter-Domain Routing) notation is used. For example, you would request a “/29” subnet from your ISP, and you would be provided with a subnet of 8 IP Addresses, 6 of which are usable – this is because 1 is the network address (the first IP) and one is the broadcast address (the last IP).  You would then allocate the remaining 6 IPs to devices, such as the gateway router, firewalls, specific NAT rules, servers, etc. Don’t worry, if necessary we can help with any IT consultancy tasks around configuration and setup.

It’s important to think about how many IPs are going to be used by ISP equipment – in an example of a leased line, if you are provided with one router, typically, that device will use one public IP address and that address will become your default gateway.  However, if you are allocated a primary and a backup leased line circuit, this will probably be deployed as a redundant router setup, using technology such as Cisco HSRP (Hot Standby Router Protocol). In this case, you will have two routers and a virtual IP address is shared between them – as you can see, this would use three IPs – therefore, if you are only allocated a /29 subnet, you would only have three remaining IPs for your own use.

Due to a global IPv4 shortage, you will need to justify any use of IPs to avoid excessive wastage – certainly, above /29, a RIPE justification will usually need to be completed.

Below is a table of CIDR subnets, their associated subnet mask, how many IPs each subnet contains (Total IPs) and how many usable IPs you can use. We have also included a “Wildcard Mask” column – this is often used on certain router configurations, particularly Cisco devices, so we’ve included that here for completeness.

If we can help with this, or if you have any questions, our IT support department are happy to talk. We supply internet connections and consult on IP address schemes, requests, etc. We’ll take you through any connectivity questions (leased lines, FTTC, ADSL+, etc.) in plain English. Give us a call on 01509 808586 to find out more.

Posted on: 25th September 2020.