1. Internet Address and Domain Name Service (DNS)
CS587x Lecture 5
Department of Computer Science
Iowa State University

2. What to cover today Internet Address Domain Name Service IPv4 CIDR
Name Resolution
Load Balancing

3. Internet Addressing IPv4 Classified IP address
Each address is represented by 4 bytes
Four numbers, 0-255, separated by dots
Classified IP address
Class+Network ID+Host ID

4. Special IP Addresses Some special addresses Reserved addresses
“this host”
“all hosts”
“localhost”
Reserved addresses
Can be used locally (behind Network Address Translator, for example)
Not routed through the Internet

5. IP Address Space Exhaustion
Restricting IP addresses to 32 bits imposes a major limitation in the number of hosts on the Internet
IPv6 calls for 128 bits address, but requires significant changes throughout much of the Internet
Some 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
Computers not visible to the Internet can share the same block of IP address

6. Classless Interdomain Routing (CIDR)
CIDR uses flexible block sizes for address allocation
CIDR allows the division between the network and host portions of the IP addresses to occur at any point in the 32-bit number
The size of a block of IP addresses could be any power of 2
A CIDR network is identified by a network address and a mask length that indicates how many bits are devoted to the network part of the address
/23: the 23-bit network address leaves 9 of the 32 bits for representing 512 hosts on that network
/16 (ISU), /24 (ISU Research Park), /13 (USWest)

7. Compatibility with Existing Addresses
The addresses allocated with class can be extended with mask
Class A address, a#.b#.c#.d# can be recognized as a#.b#.c#.d#/8
Class B address, a#.b#.c#.d# can be recognized as a#.b#.c#.d#/16
Class C address, a#.b#.c#.d# can be recognized as a#.b#.c#.d#/24

8. Advantages of CIDR
Flexible allocation of IP address blocks allows more efficient use of 32-bit address space
An organization needing 512 addresses could be assigned with a 23-bit mask, rather than an entire class B network (65536 addresses)
ISPs can now aggregate their networks into larger blocks for the purpose of routing
Suppose an ISP is assigned the /8 network. This block of addresses could be divided into smaller blocks and allocated to specific customers of this ISP
E.g., one customer can have /16 and another might have /23 network
The allocation of address blocks may depend on the size of the customer

9. Routing with CIDR Routers normally do not remember each individual IP
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
Advantage: Internet routers need only to know how to reach ISPs
Hierarchical addressing
Similar to postal office

10. 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  
Questions:
Given an IP, how to find out its hostname?
Given a hostname, how to find out its IP?

11. DNS: History 1970’s ARPANET
All host-address mappings were in hosts.txt (in /etc/hosts)
Changes were submitted to SRI-NIC by
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.txt
Internet growth was threatened
DNS was created in 1983 (RFCs 1034 and 1035), modified, updated, and enhanced by subsequent RFCs

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

13. DNS Namespace The namespace must be able to scale
Solution: 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)

14. Hierarchical Organization of Host names
root
edu
com
gov
mil
org
net fr cn
The first level names are called “Top Level Domains”
Depth of tree is arbitrary (limit 128)
No restriction on the amount of branch
Domains are subtrees
e.g. iastate.edu and cs.iastate.edu
Name collision avoided
e.g. iastate.edu and iastate.com
iastate
mit cs
eece
popeye

15. Hierarchical Administration of Host Names
root
root
edu
edu
com
com
gov
gov
mil
mil
org
org
net
net fr cn
iastate
mit
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 cs
eece
popeye

16. Domain Name Servers
Name servers, who answer “DNS” questions, are organized in hierarchies
Each 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 zone
Each 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

17. 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

18. Example of Recursive Query
root name server
popeye.cs.iastate.edu wants IP address of
1. Contacts its local DNS server,
contacts root name server, if necessary
3. Root name server contacts authoritative name server, ns1.berkeley.edu, if necessary 2 4 3 5
local name server
authorititive name server
ns1.berkeley.edu 1 6
requesting host
popeye.cs.iastate.edu

19. 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”
root name server 2
iterated queries 3 4 5
intermediate name server
(com server)
local name server 7 6 1 8
authoritative name server
requesting host
popeye.cs.iastate.edu

20. DNS query is expensive
Resolving an IP hostname may invoke several messages
Solution: 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

21. 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

22. 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

23. Example: www.akamai.com
From Ames
C:\>ping
Pinging a1440.g.akamai.net [ ] with 32 bytes of data:
Request timed out.
Ping statistics for :
Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),
From the NY area
From the UK

24. Load Balancing (2)
A query to a host name may return several IP address
e.g., may correspond to four machines with IP addresses , , ,
By default, the requesting client uses the first IP address
Heavy load on the server
DNS can vary the order of the IP addresses for each query
The response to the second query could be , , 1.2.3,4,

25. DNS Summary Internet address and CIDR
DNS is a crucial part of the internet
Namespace is hierarchical
Globally distributed and locally managed
DNS performance is enhanced by caching
DNS can help balance web server workload