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Intro to Computer Networks
DNS (Domain Name System)
Bob BradleyThe University of Tennessee at Martin
Intro to Computer Networks
DNS (Domain Name System)
The automated system used to translate computer names into equivalent IP addresses.A DNS server responds to a query by looking up the name and returning the address.DNS is a global distributed database.If you tell your web browser to connect to www.utm.edu, the web browser will use the DNS system to convert the host name (www.utm.edu) into an IP address (208.47.4.80)
Intro to Computer Networks
DNS
Converts host names to IP addresseswww.utm.edu 208.47.4.80Also does reverse translationIP Host NameAlso used for E-Mail server lookupsExample: mail for [email protected] goes to mail.utm.edu
Intro to Computer Networks
The DNS Process
Whenever a computer needs to connect to a server (such as the www.yahoo.com web server) it has to look up the server’s IP address using the DNS system.Each computer on the Internet has a list of the IP address of a local DNS server.So when a computer needs to look up an IP address, it asks its local DNS server for it.
Intro to Computer Networks
Step 1
PC
Local DNS
Root DNS
Yahoo’s DNS
www.yahoo.comWeb Server
1) P
C asks
loca
l DNS fo
r
www.yahoo
.com’s
IP
Step 1: If the PC does not already have the address for www.yahoo.com in its own DNS cache, it asks its local DNS server for the IP address information.
Intro to Computer Networks
Step 2
PC
Local DNS
Root DNS
Yahoo’s DNS
www.yahoo.comWeb Server
1) P
C asks
loca
l DNS fo
r
www.yahoo
.com’s
IP
2) Local DNS asks Root DNS
Step 2: If the local DNS server does not already have the address for www.yahoo.com in its own DNS cache, it asks the root DNS servers for the IP address of Yahoo’s DNS server.
Intro to Computer Networks
Step 3
PC
Local DNS
Root DNS
Yahoo’s DNS
www.yahoo.comWeb Server
1) P
C asks
loca
l DNS fo
r
www.yahoo
.com’s
IP
2) Local DNS asks Root DNS
3) Root returns IP of Yahoo’s DNS
Step 3: The root DNS server returns the IP address of Yahoo’s DNS server to the local DNS server.
Intro to Computer Networks
Step 4
PC
Local DNS
Root DNS
Yahoo’s DNS
www.yahoo.comWeb Server
1) P
C asks
loca
l DNS fo
r
www.yahoo
.com’s
IP
2) Local DNS asks Root DNS
3) Root returns IP of Yahoo’s DNS
4) Local DNS asks Yahoo’s DNS for www’s IP
Step 4: The local DNS server connects to Yahoo’s DNS server and asks for www.yahoo.com’s IP address.
Intro to Computer Networks
Step 5
PC
Local DNS
Root DNS
Yahoo’s DNS
www.yahoo.comWeb Server
1) P
C asks
loca
l DNS fo
r
www.yahoo
.com’s
IP
2) Local DNS asks Root DNS
3) Root returns IP of Yahoo’s DNS
4) Local DNS asks Yahoo’s DNS for www’s IP
5) Yahoo’s DNS returns IP info for www.yahoo.com
Step 5: Yahoo’s DNS server returns the IP address information of www.yahoo.com to the local DNS server.
Intro to Computer Networks
Step 6
PC
Local DNS
Root DNS
Yahoo’s DNS
www.yahoo.comWeb Server
1) P
C asks
loca
l DNS fo
r
www.yahoo
.com’s
IP
2) Local DNS asks Root DNS
3) Root returns IP of Yahoo’s DNS
4) Local DNS asks Yahoo’s DNS for www’s IP
5) Yahoo’s DNS returns IP info for www.yahoo.com
6) Loca
l DNS re
turns I
P
to PC
Step 6: The local DNS adds www.yahoo.com’s IP address to its cache and then returns the IP address info to the PC.
Intro to Computer Networks
Step 7
PC
Local DNS
Root DNS
Yahoo’s DNS
www.yahoo.comWeb Server
1) P
C asks
loca
l DNS fo
r
www.yahoo
.com’s
IP
2) Local DNS asks Root DNS
3) Root returns IP of Yahoo’s DNS
4) Local DNS asks Yahoo’s DNS for www’s IP
5) Yahoo’s DNS returns IP info for www.yahoo.com
7) PC Connects to www.yahoo.com’s IP6) L
ocal D
NS return
s IP
to PC
Step 7: The PC adds the IP address info to its local cache and can then connect to www.yahoo.com using that IP address.
Intro to Computer Networks
Local DNSRemembers
PC
Local DNS
Root DNS
Yahoo’s DNS
www.yahoo.comWeb Server
The next time your computer (or another computer on your LAN) needs to look up www.yahoo.com’s address, it will ask the local DNS server again. But this time the local DNS server will have the IP info cached and can return it directly.
Intro to Computer Networks
Intro to Computer Networks
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