1. DOMAIN NAME SYSTEM

2. Domain Name System Hostname Resolution DNS Name Lookup with DNS Domain Name Servers DNS Database Reverse Lookups

3. Hostname Resolution TCP/IP uses 32 bit address (dotted decimal) Hard for us - we use “ordinary” (canonical) names Host Name Resolution - application must find IP address corresponding to name Lookup routines provided by resolver library (gethostbyname & gethostbyaddr)

4. Solutions: –Local file (/etc/hosts) maintenance problem –Network Information System (NIS) stores host file info in master database OK for small nets (maint & distribution) –Network Information Center (NIC) HOSTS.TXT pre-1984 central site - maint & distribution problems

5. Domain Name System Hierarchy of domains –edu (Mostly US) educational institutions like universities, etc. –com Commercial organizations, companies. –org Non-commercial organizations. –net Gateways and other administrative host on a network –mil US military institutions –gov US government institutions. –Outside USA: fr - france de - Germany au - Australia

6. Name Lookup with DNS Giant distributed database Name Servers provide info on a domain or set of domains Two (or more) name servers per zone (see whois)

7. Sample Lookup Your host needs address of slotnick.psycho.havid.edu host queries names server at root domain root name server (rns) - not in my domain - in.edu domain rns contacts edu zone name server (zns) - list provided to rns rns queries one of the edu zns edu zns knows havid.edu runs a root of their own & points rns to it rns presents query for slotnik to havid.edu rns & IP address is returned info (Ipfor slotnick, havid.edu &.edu) cached for a TTL period (just in case)

8. Domain Name Servers Authoritative (Master name servers) Primary/secondary

9. DNS Database