Internet Address and Domain Name Service (DNS)

CS587x LectureDepartment of Computer Science

Iowa State University

What to cover today

Internet Address IPv4 CIDR

Domain Name Service Name Resolution Load Balancing

Internet AddressingIPv4

Each address is represented by 4 bytes Four numbers, 0-255, separated by dots

Classified IP address (developed in 1970s) Class+Network ID+Host ID

0

byte 0 byte 1 byte 2 byte 3

A

1 0B

1 1 0C

1 1 1 0D

1 1 1 1 0E

Network ID Host ID

Network ID Host ID

Network ID Host ID

Multicast

Reserved

#nets #hosts

128

16,384

2,097,152

To minimize routing table entries•Given an IP packet, a router examines its first one to four bits to determine its class and then breaks it down into network and host bits. The network bits are used to find a route to the network. Once a packet reached its target network, its host field was examined for final delivery.

16,777,216

65,536

256

networks routes

12.0.0.0 XXX

123.0.0.0 XXX

:: ::

Special IP Addresses

Some special addresses 127.0.0.1 – loopback/localhost 255.255.255.255 – broadcast

Reserved addresses Can be used locally (behind Network Address Translator, for

example) 192.168.0.0-192.168.255.255 172.16.0.0-172.31.255.255 10.0.0.0-10.255.255.255

Not routed through the Internet

Space limitation of IP Address

32 bits IPv6 calls for 128-bit address, but

requires significant changes throughout much of the Internet

Classified not flexible and efficient in address

allocation While the Internet was running out of

unassigned addresses, only 3% of the assigned addresses were actually being used

Challenges

Represent more networks Need more network IDs, given the same 32-

bits address spaceGlobal routing tables at capacity

As the number of networks on the Internet increased, so did the number of routes

A few years back, it was forecasted that the global backbone Internet routers were fast approaching their limit on the number of routes they could support.

Even using the latest router technology, the maximum theoretical routing table size is approximately 60,000 routing table entries

If nothing was done, the global routing tables would have reached capacity by mid-1994 and all Internet growth would be halted

Solutions

Assign IP to machines dynamically For an ISP, at any given time, only a small percentage

of its customers are connected to the network Not effective for DSL/cable connections, where clients

are always on

Computers not visible to the Internet can share the same block of IP address

Use those special address blocks

Classless Inter-Domain Routing (CIDR) Restructuring IP address assignments to increase

efficiency Hierarchical routing aggregation to minimize route

table entries

CIDR

A CIDR address includes the standard 32-bit IP address and also information on how many bits are used for the network prefix, e.g.,

206.13.01.48/25, the "/25" indicates the first 25 bits are used to identify the unique network leaving the remaining bits to identify the specific host

129.186.0.0/16 (ISU), 192.188.162.0/24 (ISU Research Park), 63.224.0.0/13 (USWest)

CIDR allows the division between the network and host portions of the IP addresses to occur at any point in the 32-bit number

Mask length currently is anywhere from 13 to 27 bitsCompatibility with Existing Addresses

Class A address, a#.b#.c#.d# a#.b#.c#.d#/8 Class B address, a#.b#.c#.d# a#.b#.c#.d#/16 Class C address, a#.b#.c#.d# a#.b#.c#.d#/24

Advantages of CIDRMore network IDs -- Flexible allocation of IP address blocks allows more efficient use of 32-bit address space

The size of a block of IP addresses could be any power of 2 An organization needing 512 addresses could be assigned

with a 23-bit mask, rather than an entire class B network (65536 addresses)

Minimizing routing table entries -- ISPs can now aggregate their networks into larger blocks

Suppose an ISP is assigned the 12.0.0.0/8 network, the ISP can further divide this block of addresses into smaller blocks and allocate them to its customers

E.g., one customer can have 12.45.0.0/16 and another might have 12.194.34.0/23 network

The allocation of address blocks may depend on the size of the customer

Global routers need to remember only the routes to 12.0.0.0/8

Example of Address Aggregation

ISP/Router12.0.0.0/8

Organization112.1.0.0/16

Organization212.2.0.0/16

::

Organization25512.255.0.0/16

InternetPacket with IP 12.255.1.1

networks routes

12.0.0.0/8 XXX

13.0.0.0/20 XXX

:: ::

Example of Address Aggregation

ISP/Router12.0.0.0/8

Organization112.1.0.0/16

Organization212.2.0.0/16

::

Organization25512.255.0.0/16

Internet

Packet with IP 12.255.1.1

ISP/Router12.255.0.0/16

1. Which entry to use? • The entry with longest mask length

2. Which entry to drop? • A subnet entry with longest mask

length (a block <= 8192 most likely be dropped)

• The original ISP is still responsible for resolving all addresses in its block

networks routes

12.0.0.0/8 XXX

13.0.0.0/20 XXX

12.255.0.0/16 XXX

Routing with CIDR

Internet routers normally do not remember each individual IP

Internet routers need only to know how to reach ISPs Given an IP packet, routers determine the block it

belongs to and send the packet to the ISP who are responsible for this block

The ISP needs to know how to reach each of its own separate networks

CIDR significantly reduces the growth in the number of routing table entries at each level in the network hierarchy

Currently, the global routing tables have approximately 35,000 entries

Domain Name Service

Routers need an address to route while people need a host name to remember Host Names yield information to people IP addresses yield information to routers

Solution: give each IP address a name popeye.cs.iastate.edu 129.186.3.1 www.myown.com 111.222.333.444

Questions: Given an IP, how to find out its hostname? Given a hostname, how to find out its IP?

DNS: History

1970’s ARPANET All host-address mappings were in hosts.txt (in

/etc/hosts) Changes were submitted to SRI-NIC by email New versions of hosts.txt were updated periodically from

SRI Administrators could pick names at their discretion

As the internet grew this system broke down because of

Traffic and load: SRI couldn’t handled the load Reliability: The system was unreliable since there was a

single point of contact Consistency and confliction: Names were not unique and

many hosts had inaccurate copies of hosts.txtInternet growth was threatened

DNS was created in 1983 (RFCs 1034 and 1035), modified, updated, and enhanced by subsequent RFCs

DNS Concepts

Provide a lookup mechanism for object translation (IP address hostname)DNS is implemented as a globally distributed, loosely coherent, scalable, reliable, dynamic databaseDNS consists of three components Namespace Domain Name Servers DNS queries (issued by clients)

gethostbyname() gethostbyaddr()

DNS Namespace

The namespace must be able to scaleSolution: make namespace hierarchical by naming objects based on location (within country, set of

organizations, set of companies, etc) unit within that location (company within

set of company, etc) object within unit (name of person in

company)

Hierarchical Organization of Host names

The first level names are called “Top Level Domains”Depth of tree is arbitrary (limit 128)No restriction on the amount of branchDomains are subtrees

e.g. iastate.edu and cs.iastate.edu

Name collision avoided e.g. iastate.edu and iastate.com

root

edu com gov mil org net fr cn

iastate mit

cs eece

popeye

Hierarchical Administration of Host Names

root

edu com gov mil org net fr cn

mit

cs

popeye

root

edu com gov mil org net

iastate

eece

Each zone corresponds to an administrative authority that is responsible for that portion of the hierarchy

• Zones are “administrative spaces”• Zone administrators are responsible for portion

of a domain’s name space• Authority is delegated from a parent and to a

child

Domain Name Servers

Name servers, who answer “DNS” questions, are organized in hierarchiesEach server has authority over a portion of the hierarchy

A server maintains only a subset of all names

Each server contains all the records for the hosts in its zoneEach server may know other servers who are responsible for the other portions of the hierarchy

Every server knows the root Root server knows about all top-level domains

DNS Protocol

Govern the communication between a DNS client and a DNS server

A DNS client sends a query to a DNS server, which returns a response with the requested information

DNS primarily uses UDP for sending queries and responses, although TCP may also be used

DNS queries can be Recursive : such queries request the receiving

DNS server resolve the entire request itself Iterative : such queries request the receiving

DNS server respond directly to the DNS client with the IP address of the next DNS server in the hierarchy Root server handles only iterative queries

Example of Recursive Query

popeye.cs.iastate.edu wants IP address of www.berkeley.edu

1. Contacts its local DNS server, 129.186.3.1

2. 129.186.3.1 contacts root name server, if necessary

3. Root name server contacts authoritative

name server, ns1.berkeley.edu, if

necessary requesting hostpopeye.cs.iastate.edu

www.berkeley.edu

root name server

authorititive name serverns1.berkeley.edu

local name server129.186.3.1

1

23

4

5

6

Example of Iterated Queries

Contacted server replies with name of server to contact“I don’t know this name, but you can ask this server”

requesting hostpopeye.cs.iastate.edu

www.sales.myown.com

root name server

local name server129.186.3.1

1

2

34

67

authoritative name serverwww.myown.com

intermediate name server(com server)

5

8

iterated queries

DNS query is expensive

Resolving an IP hostname may invoke several messagesSolution: caching previous query results The cached mapping can be used The cached results can be associated

with TTL to reduce the risk of using expired information

DNS-based Web Server Load Balancing

A popular web site can be replicated in different geographical locations in order to provide better service to a diverse set of clients One web site can be associated with

more than one IP addresses For example, all hosts in the acme.com may have

the same IP address The return of an IP address may depend on where a

query is sent from

Load Balancing (1)

DNS can return an IP address based on where queries come from Direct HTTP requests to a collection of

web servers that provide access to the same content

Example: www.akamai.com

From AmesC:\>ping www.akamai.comPinging a1440.g.akamai.net [209.152.119.232] with 32 bytes of data:

Request timed out.Request timed out.Request timed out.Request timed out.

Ping statistics for 209.152.119.232: Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),

From the NY area 63.240.15.146

From the UK 194.82.174.224

Load Balancing (2)A query to a host name may return several IP address e.g., www.big.com may correspond to four

machines with IP addresses 1.2.3.1, 1.2.3.2, 1.2.3.3, 1.2.3.4

By default, the requesting client uses the first IP address Heavy load on the server 1.2.3.1

DNS can vary the order of the IP addresses for each query The response to the second query could be

1.2.3.2, 1.2.3.3, 1.2.3,4, 1.2.3.1.

DNS Summary

Internet address and CIDRDNS is a crucial part of the InternetNamespace is hierarchical Globally distributed and locally managedDNS performance is enhanced by cachingDNS can help balance web server workload