3. 25.3
25-1 NAME SPACE
25-1 NAME SPACE
To be unambiguous, the names assigned to machines
To be unambiguous, the names assigned to machines
must be carefully selected from a name space with
must be carefully selected from a name space with
complete control over the binding between the names
complete control over the binding between the names
and IP addresses.
and IP addresses.
Flat Name Space
Hierarchical Name Space
Topics discussed in this section:
Topics discussed in this section:
4. 25.4
25-2 DOMAIN NAME SPACE
25-2 DOMAIN NAME SPACE
To have a hierarchical name space, a domain name
To have a hierarchical name space, a domain name
space was designed. In this design the names are
space was designed. In this design the names are
defined in an inverted-tree structure with the root at
defined in an inverted-tree structure with the root at
the top. The tree can have only 128 levels: level 0
the top. The tree can have only 128 levels: level 0
(root) to level 127.
(root) to level 127.
Label
Domain Name
Domain
Topics discussed in this section:
Topics discussed in this section:
9. 25.9
25-3 DISTRIBUTION OF NAME SPACE
25-3 DISTRIBUTION OF NAME SPACE
The information contained in the domain name space
The information contained in the domain name space
must be stored. However, it is very inefficient and also
must be stored. However, it is very inefficient and also
unreliable to have just one computer store such a huge
unreliable to have just one computer store such a huge
amount of information. In this section, we discuss the
amount of information. In this section, we discuss the
distribution of the domain name space.
distribution of the domain name space.
Hierarchy of Name Servers
Zone
Root Server
Primary and Secondary Servers
Topics discussed in this section:
Topics discussed in this section:
12. 25.12
A primary server loads all information
from the disk file; the secondary server
loads all information from
the primary server.
When the secondary downloads
information from the primary, it is called
zone transfer.
Note
13. 25.13
25-4 DNS IN THE INTERNET
25-4 DNS IN THE INTERNET
DNS is a protocol that can be used in different
DNS is a protocol that can be used in different
platforms. In the Internet, the domain name space
platforms. In the Internet, the domain name space
(tree) is divided into three different sections: generic
(tree) is divided into three different sections: generic
domains, country domains, and the inverse domain.
domains, country domains, and the inverse domain.
Generic Domains
Country Domains
Inverse Domain
Topics discussed in this section:
Topics discussed in this section:
19. 25.19
25-5 RESOLUTION
25-5 RESOLUTION
Mapping a name to an address or an address to a
Mapping a name to an address or an address to a
name is called name-address resolution.
name is called name-address resolution.
Resolver
Mapping Names to Addresses
Mapping Addresses to Names
Recursive Resolution
Caching
Topics discussed in this section:
Topics discussed in this section:
22. 25.22
25-6 DNS MESSAGES
25-6 DNS MESSAGES
DNS has two types of messages: query and response.
DNS has two types of messages: query and response.
Both types have the same format. The query message
Both types have the same format. The query message
consists of a header and question records; the
consists of a header and question records; the
response message consists of a header, question
response message consists of a header, question
records, answer records, authoritative records, and
records, answer records, authoritative records, and
additional records.
additional records.
Header
Topics discussed in this section:
Topics discussed in this section:
25. 25.25
25-7 TYPES OF RECORDS
25-7 TYPES OF RECORDS
As we saw in Section 25.6, two types of records are
As we saw in Section 25.6, two types of records are
used in DNS. The question records are used in the
used in DNS. The question records are used in the
question section of the query and response messages.
question section of the query and response messages.
The resource records are used in the answer,
The resource records are used in the answer,
authoritative, and additional information sections of
authoritative, and additional information sections of
the response message.
the response message.
Question Record
Resource Record
Topics discussed in this section:
Topics discussed in this section:
26. 25.26
25-8 REGISTRARS
25-8 REGISTRARS
How are new domains added to DNS? This is done
How are new domains added to DNS? This is done
through a registrar, a commercial entity accredited by
through a registrar, a commercial entity accredited by
ICANN. A registrar first verifies that the requested
ICANN. A registrar first verifies that the requested
domain name is unique and then enters it into the
domain name is unique and then enters it into the
DNS database. A fee is charged.
DNS database. A fee is charged.
27. 25.27
25-9 DYNAMIC DOMAIN NAME
25-9 DYNAMIC DOMAIN NAME
SYSTEM (DDNS)
SYSTEM (DDNS)
The DNS master file must be updated dynamically.
The DNS master file must be updated dynamically.
The
The Dynamic Domain Name System (DDNS)
Dynamic Domain Name System (DDNS) therefore
therefore
was devised to respond to this need. In DDNS, when a
was devised to respond to this need. In DDNS, when a
binding between a name and an address is determined,
binding between a name and an address is determined,
the information is sent, usually by DHCP to a primary
the information is sent, usually by DHCP to a primary
DNS server. The primary server updates the zone. The
DNS server. The primary server updates the zone. The
secondary servers are notified either actively or
secondary servers are notified either actively or
passively.
passively.
28. 25.28
25-10 ENCAPSULATION
25-10 ENCAPSULATION
DNS can use either UDP or TCP. In both cases the
DNS can use either UDP or TCP. In both cases the
well-known port used by the server is port 53. UDP is
well-known port used by the server is port 53. UDP is
used when the size of the response message is less than
used when the size of the response message is less than
512 bytes because most UDP packages have a 512-byte
512 bytes because most UDP packages have a 512-byte
packet size limit. If the size of the response message is
packet size limit. If the size of the response message is
more than 512 bytes, a TCP connection is used.
more than 512 bytes, a TCP connection is used.
29. 25.29
DNS can use the services of UDP or TCP
using the well-known port 53.
Note
