1. ccTLD/us
http://www.courts.state.co.us/ というような使い方があるらしい。 （co.us は Colorado 関係のために存在しているようだ）
co.us はゾーンではない。 https://en.wikipedia.org/wiki/List_of_courts_of_the_United_States からたどれる。
$ dnsq ns co.us a.root-servers.net
2 co.us: 305 bytes, 1+0+6+9 records, response, noerror query: 2 co.us authority: us 172800 NS a.cctld.us authority: us 172800 NS b.cctld.us authority: us 172800 NS c.cctld.us authority: us 172800 NS e.cctld.us authority: us 172800 NS f.cctld.us authority: us 172800 NS k.cctld.us additional: a.cctld.us 172800 A 156.154.124.70 additional: b.cctld.us 172800 A 156.154.125.70 additional: c.cctld.us 172800 A 156.154.127.70 additional: e.cctld.us 172800 A 156.154.126.70 additional: f.cctld.us 172800 A 209.173.58.70 additional: k.cctld.us 172800 A 156.154.128.70 additional: b.cctld.us 172800 AAAA 2001:503:d1ae:ffff:ffff:ffff:ffff:ff7e additional: f.cctld.us 172800 AAAA 2001:500:3682::11 additional: k.cctld.us 172800 AAAA 2001:503:e239::3:1
$ dnsq ns co.us 156.154.124.70
2 co.us: 89 bytes, 1+0+1+0 records, response, authoritative, noerror query: 2 co.us authority: us 900 SOA a.cctld.us hostmaster.neustar.biz 2013096808 900 900 604800 86400
2. state.co.us
$ dnsq ns state.co.us a.cctld.us
2 state.co.us: 99 bytes, 1+0+2+2 records, response, noerror query: 2 state.co.us authority: state.co.us 900 NS dns2.state.co.us authority: state.co.us 900 NS dns1.state.co.us additional: dns1.state.co.us 7200 A 192.70.175.110 additional: dns2.state.co.us 7200 A 165.127.20.21
NS TTL が短い。
$ dnsq ns courts.state.co.us dns1.state.co.us
2 courts.state.co.us: 106 bytes, 1+2+0+2 records, response, authoritative, noerror query: 2 courts.state.co.us answer: courts.state.co.us 3600 NS dns1.state.co.us answer: courts.state.co.us 3600 NS dns2.state.co.us additional: dns1.state.co.us 86400 A 192.70.175.110 additional: dns2.state.co.us 86400 A 165.127.20.21
courts zone は同居。