If you only want to have records for b.c
and a.b.c
subdomains are not necessary. It's completely valid to have more sub-levels in one file, for example in zonefile for c
you can have
b IN A 127.0.0.1
a.b IN A 127.0.0.2
If you really want a subdomain b.c
first define delegation i c
zonefile.
b IN NS ns.with.b.c.zone
the ns.with.b.c.zone
can point on the same server, or to hosting provider for example. Is good practice to have at least two DNS server for domain. RFC1912
next define A
record for b.c
in zonefile for b.c
@ IN A 127.0.0.2
or
b.c. IN A 127.0.0.2