Why 63 bytes of limit on dns label imposed?

Question

The DNS rfc 1035 imposes a limit of 63 byte on each of DNS label as well as the total length of DNS name should not exceed 255 bytes. Why 63 bytes of limit on dns label imposed? How can system support the unlimited label length?

Thanks.

Answer 1

The 63-byte limit is because in the DNS protocol, labels stored as <length><label>, length is a single byte, but two high bits of the length field reserved for something else (compression) thus leaving 6 bits for the length itself, 2^6=64 possible values - 0..63.

System can not support unlimited label length. It's not an artificial limit, it's interwoven into the protocol.