Got it - just do RTFM
FULL EXTRACT FROM RC.CONF(5) MAN PAGE:
It also possible to configure multiple IP addresses in Class-
less Inter-Domain Routing (CIDR) address notation, whose each
address component can be a range like inet 192.0.2.5-23/24 or
inet6 2001:db8:1-f::1/64. This notation allows address and
prefix length part only, not the other address modifiers.
Note that the maximum number of the generated addresses from
a range specification is limited to an integer value speci-
fied in netif_ipexpand_max in rc.conf(5) because a small typo
can unexpectedly generate a large number of addresses. The
default value is 2048. It can be increased by adding the
following line into rc.conf(5):
netif_ipexpand_max="4096"
In the case of 192.0.2.5-23/24, the address 192.0.2.5 will be
configured with the netmask /24 and the addresses 192.0.2.6
to 192.0.2.23 with the non-conflicting netmask /32 as
explained in the ifconfig(8) alias section. Note that this
special netmask handling is only for inet, not for the other
address families such as inet6.
With the interface in question being ed0, an example could
look like:
ifconfig_ed0_alias2="inet 192.0.2.129/27"
ifconfig_ed0_alias3="inet 192.0.2.1-5/28"
and so on.
Note that ipv4_addrs_<interface> variable was supported for
IPv4 CIDR address notation. It is now deprecated because the
functionality was integrated into
ifconfig_<interface>_alias<n> though ipv4_addrs_<interface>
is still supported for backward compatibility.
For each ifconfig_<interface>_alias<n> entry with an address
family keyword, its contents are passed to ifconfig(8). Exe-
cution stops at the first unsuccessful access, so if some-
thing like this is present:
ifconfig_ed0_alias0="inet 127.0.0.251 netmask 0xffffffff"
ifconfig_ed0_alias1="inet 127.0.0.252 netmask 0xffffffff"
ifconfig_ed0_alias2="inet 127.0.0.253 netmask 0xffffffff"
ifconfig_ed0_alias4="inet 127.0.0.254 netmask 0xffffffff"
Then note that alias4 would not be added since the search
would stop with the missing ``alias3'' entry. Because of
this difficult to manage behavior, there is
ifconfig_<interface>_aliases variable, which has the same
functionality as ifconfig_<interface>_alias<n> and can have
all of entries in a variable like the following:
ifconfig_ed0_aliases="\
inet 127.0.0.251 netmask 0xffffffff \
inet 127.0.0.252 netmask 0xffffffff \
inet 127.0.0.253 netmask 0xffffffff \
inet 127.0.0.254 netmask 0xffffffff"
It also supports CIDR notation.
