3 ms·

I can never remember whether /x means "the last x bits are 0" or "the first x bits are 1" People should write 80/48 or 48/80 to be clear

by dheera 2y ago

patmorgan23 2y ago
/x is almost always the number of network bits (so the first half). There are some Cisco ISO commands that are the opposite but those are by far the minority. 99/100 it means the first bits.
merlincorey 2y ago
> I can never remember whether /x means "the last x bits are 0" or "the first x bits are 1" > People should write 80/48 or 48/80 to be clear The clarity is found implied in your preferred example. - "80/" would mean "80 bits before" - "/48" would mean "48 bits after"
- dheera 2y ago
  ... and this is the opposite of the other 2 responses
immibis 2y ago
It's not about how many bits are 1 - it's about how many bits are important. And the first bits are always most important. So it's the first x bits. If you have a /48 then 48 bits are used to determine the address is yours. Any address which matches in the first 48 bits is yours. If you have a /64, any address which matches in the first 64 bits is yours.
- patmorgan23 2y ago
  It's about how many bits are 1, in the subnet mask.
  - immibis 2y ago
    The number of bits that are important is the number of 1 bits in the which bits are important mask, yes. I thought you couldn't remember how that mask worked.
aftbit 2y ago
/48 is netmask of ffff:ffff:ffff:0:0:0:0:0. `sipcalc` can help with this. $ sipcalc ::/48 -[ipv6 : ::/48] - 0 [IPV6 INFO] Expanded Address - 0000:0000:0000:0000:0000:0000:0000:0000 Compressed address - :: Subnet prefix (masked) - 0:0:0:0:0:0:0:0/48 Address ID (masked) - 0:0:0:0:0:0:0:0/48 Prefix address - ffff:ffff:ffff:0:0:0:0:0 Prefix length - 48 Address type - Reserved Comment - Unspecified Network range - 0000:0000:0000:0000:0000:0000:0000:0000 - 0000:0000:0000:ffff:ffff:ffff:ffff:ffff I remember how this works because of the IPv4 examples that I have baked into my head, e.g. 10.0.0.0/8 or 192.168.1.0/24. Clearly the first 24 bits must be 1 for that last one to make any sense. I recently found a case where an "inverted" netmask makes sense - when you want to allow access through a firewall to a given IPv6 host (with auto-config address) regardless of the network that your provider has assigned.

I can never remember whether /x means "the last x bits are 0" or "the first x bits are 1" People should write 80/48 or 48/80 to be clear

by dheera 2y ago

I can never remember whether /x means "the last x bits are 0" or "the first x bits are 1" People should write 80/48 or 48/80 to be clear

patmorgan23 2y ago

/x is almost always the number of network bits (so the first half). There are some Cisco ISO commands that are the opposite but those are by far the minority. 99/100 it means the first bits.

merlincorey 2y ago

> I can never remember whether /x means "the last x bits are 0" or "the first x bits are 1" > People should write 80/48 or 48/80 to be clear The clarity is found implied in your preferred example. - "80/" would mean "80 bits before" - "/48" would mean "48 bits after"

dheera 2y ago
... and this is the opposite of the other 2 responses

immibis 2y ago

It's not about how many bits are 1 - it's about how many bits are important. And the first bits are always most important. So it's the first x bits. If you have a /48 then 48 bits are used to determine the address is yours. Any address which matches in the first 48 bits is yours. If you have a /64, any address which matches in the first 64 bits is yours.

patmorgan23 2y ago
It's about how many bits are 1, in the subnet mask.
- immibis 2y ago
  The number of bits that are important is the number of 1 bits in the which bits are important mask, yes. I thought you couldn't remember how that mask worked.

aftbit 2y ago

/48 is netmask of ffff:ffff:ffff:0:0:0:0:0. `sipcalc` can help with this. $ sipcalc ::/48 -[ipv6 : ::/48] - 0 [IPV6 INFO] Expanded Address - 0000:0000:0000:0000:0000:0000:0000:0000 Compressed address - :: Subnet prefix (masked) - 0:0:0:0:0:0:0:0/48 Address ID (masked) - 0:0:0:0:0:0:0:0/48 Prefix address - ffff:ffff:ffff:0:0:0:0:0 Prefix length - 48 Address type - Reserved Comment - Unspecified Network range - 0000:0000:0000:0000:0000:0000:0000:0000 - 0000:0000:0000:ffff:ffff:ffff:ffff:ffff I remember how this works because of the IPv4 examples that I have baked into my head, e.g. 10.0.0.0/8 or 192.168.1.0/24. Clearly the first 24 bits must be 1 for that last one to make any sense. I recently found a case where an "inverted" netmask makes sense - when you want to allow access through a firewall to a given IPv6 host (with auto-config address) regardless of the network that your provider has assigned.