I have a very basic question about IPv6 prefix. Taking a very simple example about prefix notation:


My question is: Do we call the whole thing under quote as prefix or just the value after back slash?

In general term, I understand that prefix is a way to tell how many bits of the IPv6 address is actually the network part. In above example, it is clearly that first 64 bits (8 octets) are Network address and remaining 64 bits(8 octets) will be Host Identifier. But, to be precise, when we talk about prefix, do we end up calling that number after back slash(i.e 64 in above example) as prefix or the whole thing (entire thing under double quotes)as prefix ? I would like to get 100% clear about the terminology and its interpretation? Please share your kind response.

  • 9
    The number after / is called prefix length
    – manish ma
    Commented Nov 23, 2021 at 14:15
  • 1
    Network address and prefix are the very same thing.
    – Zac67
    Commented Nov 23, 2021 at 15:46
  • 2
    This question, of course, applies equally well to IPv4 prefixes, e.g. There is nothing different for IPv6 or IPv4 prefix terminology. The difference in terminology is about IPv4 octets and IPv6 hexadecimal, variously called words, 16-bit fields, hexadecimal fields or parts, etc. in various RFCs.
    – Ron Maupin
    Commented Nov 23, 2021 at 15:50
  • 9
    Nitpick: "/" is a "forward slash" (or just "slash"). "\" is a "back slash". Commented Nov 24, 2021 at 9:40

1 Answer 1


From RFC4291 section 2.3:

An IPv6 address prefix is represented by the notation:


The whole thing is called prefix.

First part up to / is called address.

Second part after / is called prefix-length.

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