You need an array of 64-bit values: a 32-bit end address, and a 32-bit ASN. Use 0 for unallocated ranges - it's an invalid ASN.
For IPv6 you need more. It just barely fits if you consider that all public Internet addresses start with 001 and only up to 48 bits can be a published prefix - that's 45 bits - and the public ASNs go up to 19 bits according to your article - that's exactly 64 in total (those high ASNs are bad data from wherever you're getting it from). But it won't work once the next 100000 ASNs get assigned, so you'd better go up to 96 or 128 bits and store the ASN properly in 32 bits.
For cache efficiency you should have one table of addresses and another parallel table of ASNs. Then every access to the address table that pulls in a cache line doesn't waste half the cache line with ASNs you aren't looking for. It won't affect the total memory use, however.
A separate table would hold the name and country for each ASN.
For IPv6 you need more. It just barely fits if you consider that all public Internet addresses start with 001 and only up to 48 bits can be a published prefix - that's 45 bits - and the public ASNs go up to 19 bits according to your article - that's exactly 64 in total (those high ASNs are bad data from wherever you're getting it from). But it won't work once the next 100000 ASNs get assigned, so you'd better go up to 96 or 128 bits and store the ASN properly in 32 bits.
For cache efficiency you should have one table of addresses and another parallel table of ASNs. Then every access to the address table that pulls in a cache line doesn't waste half the cache line with ASNs you aren't looking for. It won't affect the total memory use, however.
A separate table would hold the name and country for each ASN.