In v6, /48 is the minimum prefix that can be advertised by the routers in the Internet. I believe in v4, its /24. Until yesterday I wasn’t aware of that. I might have thought you could even advertise one IP (ie. /128 or /32) but that’s not possible. Ref: https://blog.apnic.net/2020/06/01/why-is-a-48-the-recommended-minimum-prefix-size-for-routing

There are tools like https://tehnoblog.org/ip-tools/ip-address-in-cidr-range/ which does the job. It doesn’t support IPv6. One of my colleagues today taught me how to do it by hand quickly. Say we have an IP 2b06:4600:1101:0:abcd:efa:dbd:ea60:f5a6 is in the CIDR 2b06:4600:1101::/64 First step is to check what’s the mask bits. Here it is 64. So you take the address, take the first 64 bits and see if they are the same as the CIDR.

This article is part of my Today I Learnt. I have been trying to share whatever I learnt newly on the day. Some of them are basic, but still I want to write it down for two reasons. 1. To help me understand the concepts better 2. Potential help other people who running in to same issue. Alright, It’s all started when I bought a new domain (synergy.net) for my Dad.