Approximate setup:
Host iface has 2001:db8:1000::1/48 and 203.0.113.113/32,
host epair has 2001:db8:1000:5::1/64,
jail epair has 2001:db8:1000:5::3/64.
On host with epair "host":
# ifconfig epair create
# ifconfig epair0a name host
# ifconfig epair0b name jail
# ifconfig host inet6 2001:db8:1000:5::1/64 auto_linklocal
# ipfw nat64lsn NAT64 create prefix4 203.0.113.113/32 prefix6 64:ff9b::/96
# (search stops at the nat64lsn rule, place these after IPv4 or IPv6 rules:)
# ipfw add (index) nat64lsn NAT64 all from any to 203.0.113.113/32 in
# ipfw add (index) nat64lsn NAT64 all from 2001:db8:1000::/48 to 64:ff9b::/96 in
# sysctl net.inet.ip.fw.nat64_direct_output=1
# sysctl net.inet6.ip6.forwarding=1
# sysctl net.inet.ip.forwarding=1
On VNET-enabled jail with epair "jail":
# ifconfig jail inet6 2001:db8:1000:5::3/64 auto_linklocal prefer_source
# route -6 add default 2001:db8:1000:5::1
# ifconfig jail 192.168.1.1/32
# ifconfig jail inet6 2001:db8:1000:5::c0a8:101/64
# route -4 add default 192.168.1.1
# sysctl net.inet.ip.fw.nat64_direct_output=1
# ipfw nat64clat CLAT create clat_prefix 2001:db8:1000:5::/96 plat_prefix 64:ff9b::/96 local
# ipfw add 10 nat64clat CLAT ip4 from 192.168.1.1/32 to any out
# ipfw add 11 nat64clat CLAT ip6 from 64:ff9b::/96 to 2001:db8:1000:5::/96 in
# ipfw add 20 pass all from any to any
# telnet 1.1.1.1 80