How to convert size represented in one prefix length to another?

Question

Example, let's say if someone says, I have allocated one /64 address. How can I convert that to a size in /30? How many /30 equals to one /64?

Answer 1

How can I convert that to a size in /30?

This question does not actually make sense.

How many /30 equals to one /64?

Neither does this question make sense.

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I assume you mean IPv6 because there is no such thing as an IPv4 /64 network.

Your last question could make sense if you reverse it to ask how many /64 networks can be made from a /30 prefix. The way to calculate that is to subtract 30 from 64 to get 34. The answer would be 2 to the power of the result.

64 - 30 = 34
2^34 = 17,179,869,184

That means you can derive 17,179,869,184 /64 networks from one /30 prefix.

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For IPv6, you will probably divide your /30 prefix into /48 (or shorter) prefixes to assign each site a /48 prefix. That means that you can have 262,144 sites, each with 65,536 standard IPv6 /64 networks.

The reason to use /48 (or shorter) for each site is that is the longest prefix that the ISPs will advertise on the public Internet.