1. Chabot College
ELEC 99.08
Name Resolution

2. Name Resolution Topics
Concepts
name to IP address mapping
“fully qualified” domain names
Methods
host tables
domain name services (DNS)
Commands
ip host
ip domain-lookup
ip name-server
ip domain-name
show hosts

3. Name Resolution Purposes
1. Convenience of humans.
Which is easier to remember?

4. Name Resolution Purposes
2. Ability to move machines with no disruption of access.
What happens when we move a server to a different network or subnet? How do users find it?
becomes
but
remains

5. Fully Qualified Domain Name
Unique in the world - no duplicates.
Example: library.stanford.edu
Heirarchical structure:
Top level domain (e.g. edu)
Domain (e.g. stanford)
Host (e.g. library)

6. Fully Qualified Domain Name
chabotde.clpccd.cc.ca.us
Host name
Domain name
Top-Level Domain
Sub-Domains

7. Fully Qualified Domain Name
For our labs, we’ll assume this domain name has been assigned: lab.edu
Our router’s fully qualified domain names will therefore be:
fremont.lab.edu
hayward.lab.edu
oakland.lab.edu

8. Default Domain Name
TCP/IP configurations include a default domain to be appended to any unqualified hostname (e.g. hayward).
For our labs, we’ll use this default domain: lab.edu
Then when you ping fremont, TCP/IP will actually ping fremont.lab.edu
Saves a lot of typing!

9. Resolving Names to Addresses
Host Tables
Domain Name Services

10. Host Table Created manually on each router or computer
Maps names to IP addresses
fremont.lab.edu
hayward.lab.edu
Provides name resolution only on the host on which the table was created.
Fast, low overhead, cached in RAM.

11. Host Table Commands ip host [name] [address]
May include up to 8 addresses per name
Global config mode
oak(config)#ip host fremont
oak(config)#ip host hayward
oak(config)#ip host oakland

12. Domain Name Services (DNS)
A network service
Runs on one or two servers that are used by all hosts on the network
Maps names to IP addresses
fremont.lab.edu
hayward.lab.edu
Used throughout the global Internet

13. DNS - Special Considerations
Billions of IP addresses in use, many mapped to a name. (We could never fit them in a hosts file.)
IP addresses and domain names change daily, and new ones are assigned daily. (We could never keep hosts files up to date.)
DNS is like a global phone book that must be
kept up-to-date
very fast in its response

14. How DNS Works
Local host sends DNS query to the local name server defined in the host’s TCP/IP config. (UDP port 53)
If the local name server knows the address of the target name, it sends back a reply.
Otherwise, it forwards the query to one of the 13 root name servers on the Internet.
The root server then returns the IP address of the primary name server responsible for the domain being requested.
The local name server then begins a recursive process, contacting the primary name server and then the authoritative name server for the target domain.
The local name server then fills the local host’s request for the target ip address.
The host then uses the supplied IP address to contact its target.

15. How DNS Works - Example
To find the IP address of technet.micrsoft.com...
your PC asks the Chabot name server, which then performs the following steps:
Asks one of the 13 root name servers on the Internet for the addresses of the com name servers
Asks one of these com servers for the address of the microsoft.com name server
Ask the microsoft.com name server for the address of the host: technet.microsoft.com.
The chabot name server then replies to your PC with the target IP address

16. How DNS Works

17. DNS Commands no ip domain-lookup
Turns off the use of DNS to look up the ip address mapped to a hostname
Used in our practice lab to prevent delays due to mistyped commands
Global config mode
oak(config)#no ip domain-lookup
oak(config)#

18. DNS Commands ip domain-lookup
Turns on the use of DNS to look up the ip address mapped to a hostname
This is the default for all routers, so this command is only needed if lookup was previously turned off with no ip domain-lookup
Global config mode
oak(config)#ip domain-lookup
oak(config)#

19. DNS Commands ip name-server xxx.xxx.xxx.xxx
Sets the address for the DNS server to be used
All DNS queries will go to this server
Up to six server addresses may be listed, separated by spaces
Global config mode
oak(config)#ip name-server
oak(config)#

20. DNS Commands ip domain-name [name]
Sets the default domain to be appended to “unqualified” names
Saves repetitive typing of local domain
Global config mode
oak(config)#ip domain-name lab.edu
oak(config)#

21. DNS Commands Typical configuration Global config mode
oak(config)#ip domain-lookup
oak(config)#ip name-server
oak(config)#ip domain-name lab.edu
oak(config)#
On a Windows PC, the same settings are made in the Network Control Panel, TCP/IP properties.

22. Show Hosts Commands show hosts
Lists the host-address mappings cached in RAM
privileged EXEC mode
router#show hosts
Default domain is CISCO.COM Name/address lookup uses domain service Name servers are
Host Flag Age Type Address(es)
SLAG.CISCO.COM (perm, OK) IP
CHAR.CISCO.COM (temp, OK) IP
CHAOS.CISCO.COM (temp, OK) IP
DIRT.CISCO.COM (temp, EX) IP
DUSTBIN.CISCO.COM (temp, EX) IP
DREGS.CISCO.COM (temp, EX) IP

23. Show Hosts Commands Flags: perm - address resolved by host table
temp - address resolved by DNS
router#show hosts
Default domain is CISCO.COM Name/address lookup uses domain service Name servers are
Host Flag Age Type Address(es)
SLAG.CISCO.COM (perm, OK) IP
CHAR.CISCO.COM (temp, OK) IP
CHAOS.CISCO.COM (temp, OK) IP
DIRT.CISCO.COM (temp, OK) IP
DUSTBIN.CISCO.COM (temp, EX) IP
DREGS.CISCO.COM (temp, EX) IP

24. DNS Server Records A few DNS record types
A (host address)
CNAME (alias - cannonical name)
PTR (domain name pointer - for reverse lookup)
MX (mail exchange - mail server for domain)
CNAME records mean that a host at one IP address can have many names.

25. Let’s try it! Point your browser at: http://www.clpccd.cc.ca.us
For the host at , Chabot’s DNS servers list
porter as the host name (A-record)
www as a name alias (CNAME-record)

26. Let’s try it! Point your browser at: http://porter
On your PC, the default domain is set to
clpccd.cc.ca.us
Therefore, when you browse to porter, you reach the ip address of porter.clpccd.cc.ca.us

27. Let’s try it! At the DOS prompt: ping davis
On your PC, the default domain is set to
clpccd.cc.ca.us
Therefore, when you ping davis, you reach the ip address of davis.clpccd.cc.ca.us

28. Lookup Sequence 1. Check cache in RAM. If entry not found,
2. Check hosts table. If entry not found,
3. Query local DNS server. If entry not found, refer to root server and then to authoritative DNS server for the target domain.
Therefore, host table entries will be used first to satisfy a query.

29. Site Blocking - a simple approach...
What does this address mean?
“loopback” address - your own NIC

30. Site Blocking - a simple approach...
Suppose I wanted to prevent my browser from diplaying ads from ad1.doubleclick.net which DNS resolves to
Where would my browser go if my PC’s host table contained this entry? ad1.doubleclick.net
Take a look at this hosts file, which is designed to support ad blocking.
For more info, point your browswer at