Nameserver records outside current domain

[Bob Puff@NLE]

Hello,

Was looking at my DNS records, and had a question:

In a multiple-domain hosting environment, where the primary domain is foo.com.  The entry for bar.com has:
@	IN	NS	ns1.foo.com
@	IN	NS	ns2.foo.com

Now since all the other records in this bar.com domain all resolve to IP addresses, how does the NS records resolve to an IP?  Does it have to do another lookup for foo.com to find these?

Bob