[I'm using a /60 prefix in this example just for ease of computation/display. A /48 prefix should be just the same. Also, I'm not sure why my IPv6 address examples are displayed with an icon in the middle...ignore the icon.]
Assuming you create a dynamic pool from a DHCPv6 client request to your ISP like the followiing:
/ipv6 dhcp-client /add interface = WAN add-default-route=yes pool-name=ipv6_on_wan request=address,prefix prefix-hint=::/60
Then, assuming the DHCPv6 request from the ISP succeeds, a dynamic IPv6 pool will be created. Suppose the prefix received from the ISP is
2601:601
5e70::/60. Then the pool prefix will be 2601:601
5e70::/60 and the pool's prefix length parameter will be set to 64 by default. The pool will look like this:
/ipv6/pool/print
Flags: D - DYNAMIC
Columns: NAME, PREFIX, PREFIX-LENGTH, EXPIRES-AFTER
# NAME PREFIX PREFIX-LENGTH EXPIRES-AFTER
0 D ipv6_on_wan 2601:601:1100:5e70::/60 64 2d22h29m2s
Now, every time you assign an IPv6 address from the pool, the prefix will increment by 1. So
/ipv6 address add interface=ether1 from-pool=ipv6_on_wan address=::1/64
/ipv6 address add interface=ether2 from-pool=ipv6_on_wan address=::1/64
/ipv6 address add interface=ether3 from-pool=ipv6_on_wan address=::1/64
will assign address
2601:601
5e70::1/64 to ether1 and
2601:601
5e71::1/64 to ether2 and
2601:601
5e72::1/64 to ether3
The prefix part of the address comes from the pool (which is configured to hand out /64 prefixes) and the host part of the address comes from the address= parameter.
You can keep handing out new addresses (and new subnets) in this manner until the pool is exhausted, which in this case of a /60 prefix from the ISP would be 16 /64 subnets. With a /48 from your ISP you could hand out...well a lot more subnets.
Alternatively, if you wanted to hand out /62 subnets rather than /64 subnets, you could create the pool with the parameter "pool-prefix-length=62". In this example, with a /60 allocation from your ISP, you would only be able to hand out 4 /62 subnets.
Now, every time you assign an IPv6 address from the pool, the prefix will increment by 4. So
/ipv6 address add interface=ether1 from-pool=ipv6_on_wan address=::1/62
/ipv6 address add interface=ether2 from-pool=ipv6_on_wan address=::1/62
/ipv6 address add interface=ether3 from-pool=ipv6_on_wan address=::1/62
will assign address
2601:601
5e70::1/62 to ether1 and
2601:601
5e74::1/62 to ether2 and
2601:601
5e78::1/62 to ether3