Understanding DNS: From Names to IP Addresses, Slides of Computer Networks

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2: Application Layer 1

5: DNS

2: Application Layer 2

Names and IP addresses

People: many identifiers:  SSN, name, Passport #

Internet hosts, routers: many identifiers too  IP address (32 bit) - used for addressing datagrams  “name”, e.g., www.google.org - used by humans

Q: map between IP addresses and name?

DNS does

..but before we talk about DNS lets talk more about names and addresses!

2: Application Layer 4

Mapping Not 1 to 1

 One name may map to more than one IP

address

 IP addresses are per network interface  Multihomed machines have more than one network interface - each with its own IP address  Example: routers must be like this

 One IP address may map to more than one

name

 One server machine may be the web server (www.foo,com), mail server (mail.foo.com)etc.

2: Application Layer 5

How to get names and

numbers?

 Acquisition of Names and numbers are both

regulated

 Why?

2: Application Layer 7

Domain name structure

ccTLDs

root (unnamed)

com edu gov mil net org ... fr gr us uk ...

google^ ustreas^ second level (sub-)domains

gTLDs

gTLDs= Generic Top Level Domains ccTLDs = Country Code Top Level Domains

2: Application Layer 8

Top-level Domains (TLDs)

 Generic Top Level Domains (gTLDs)

 .com - commercial organizations  .org - not-for-profit organizations  .edu - educational organizations  .mil - military organizations  .gov - governmental organizations  .net - network service providers  Newer: .biz, .info, .name, …

 Country code Top Level Domains (ccTLDs)

 One for each country

 Most popular domain is com, then de

2: Application Layer 10

Want to be a registrar?

 From ICANN (2012):

http://www.icann.org/en/resources/regis

trars/accreditation

 Application + $3500 application fee

 Sign agreement

 Demonstrate $70,000 in working capital

 Yearly fee - $4000 for first TLD + $

for each additional

2: Application Layer 11

How to get an IP Address?

 Answer 1: Normally, answer is get an IP

address from your upstream provider

 This is essential to maintain efficient routing!

 Answer 2: If you need lots of IP addresses

then you can acquire your own block of

them.

 Get them from a regional Internet registry

• 2: Application LayerDocsity.com

2: Application Layer 14

Obtaining a Block of IPv

addresses

 Price (ARIN,Sept 2009)  https://www.arin.net/fees/fee_schedule.html  $2250/year for /20 or /19 ; $18000/year for a /13 or larger (initial fee for first year doubled)  /20 = 20 of the 32 bits in IP address are specified, 12 bits free, ~2 12 = 4096 possible hosts  See why a /13 would be more expensive than a /20?

 Can’t just pay and not use them  IP address space is a scarce resource  You must prove you have fully utilized a small block before can ask for a larger one!

2: Application Layer 16

Mapping from name to IP Address?

How could we provide this service?  In the beginning, file containing mapping for all hosts copied to each new host  Size of file?  Propagation of changes?  Centralized DNS server?  single point of failure  traffic volume  distant centralized database  maintenance

doesn’t scale!

 no server has all name-to-IP address mappings

2: Application Layer 17

DNS: Domain Name System

Domain Name System:

 distributed database implemented in hierarchy of

many name servers

 application-layer protocol host, routers, name

servers to communicate to resolve names

(address/name translation)  note: core Internet function implemented as application-layer protocol  complexity at network’s “edge”

2: Application Layer 19

Mapping Name Servers to

“Zones”

root

lucent clarkson

com gov edu ...

ustreas

bep

www

irs IRS NS

Lucent NS Ustreas NS

Root NS

2: Application Layer 20

Kinds of Name Servers

Name server: process running on a host that processes DNS requests  local name servers:

• each ISP, company haslocal (default) name server
• host DNS query first goes to local name server  authoritative name server:
• can perform name/address translation for a specific domain or zone  root name server:
• Knows the authoritative server for each domain  intermediate name server:
• Authoritative servers for a large domain may hand off queries to lower level name servers that are responsible for a portion of the domain