URLs
The Uniform Resource Locator (URL) is used to provide a
human recognisable address. In the case of web pages this usually begins with
www. This may be preceded by http://, indicating that the Hyper-Text Transfer
Protocol (HTTP) is being used.
The URL system is hierarchically organised. The Top Level
Domain (TLD) is on the far right of the address, as you move to the left the
address information is more localised. The second level of domain names may
repeat some of the TLD names where the TLD is a country code. e.g.: physiology.ucl.edu.uk, mit.edu; stmartinoftours.org.uk,
iana.org. The second level domain name, co is used on the second
level in place of com for companies, e.g. achme.co.uk.
| Top Level Domain
(TLD) Names |
Original Global |
com, org, net, edu,
int |
| |
Recently Added Global |
biz, info, name, museum,
aero, pro |
| |
USA only |
mil, gov |
| |
Country code domains |
country codes, e.g.
.uk, .it, .no, .jp, etc. – for UK, Italy, Norway, Japan, etc. |
| |
Infrastructure |
.arpa |
The list of allowable TLD names is evolving, see www.internic.com for the latest information
about this.
Assigning IP numbers for the top level domains is administered
by a central body, the Internet Assigned Numbers Authority (IANA), www.iana.org.
Each of the TLD names is administered by a different body, with the overall
co-ordination residing with the IANA; with the country code TLD names maintained
by the IANA.
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Address Resolution Via DNS
The DNS constitutes a worldwide distributed database of
Internet domains and host names. Nameservers handle sections of the database
called zones. Whenever a request is made for a URL address, the request
goes to the local DNS server. If the address is held by the local DNS then
it can return it. If it is not known then the request is passed on to another
DNS and so on until the address is found or it times out.
The resolution of addresses via the DNS scheme is segmented
such that each part of the address is resolved in turn, possibly by a number
of DNS servers. The segmentation of the database allows for this, spreading
the work. It is this segmentation that makes the database of Internet domains
distributed.
Normally the zone data is held in lookup tables on nameserver
machines. Under this system, when a change is required because a domain address
is to be altered, the administrator needs to manually edit the files and reload
them. The administrator need only administer those domains within his zone.
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Dynamic Host Configuration Protocol (DHCP)
and DNS
Dynamic Host Configuration Protocol (DHCP) allows for the
dynamic allocation of IP addresses. This saves manually assigning every machine
a unique IP address. Every time a machine logs on to the local network, the
local DHCP server assigned that machine a temporary IP address. The DHCP server
has a list of allowable IP addresses that it is able to assign to machines.
When a machine logs onto the local network, if DHCP is being used, the machine
will request an IP address from the DHCP server, which will assign an IP address
from its list of allowable addresses.
Only part of the actual IP address will be dynamic, typically
the final IP number.
Some nodes may require static
IP addresses. This may especially be the case for routers, servers and printers.
In such cases DHCP is not used. It is possible to use a combination of static
IP addresses DHCP. For the majority of computers dynamic addresses are perfectly
alright. Whether DHCP is used or static (manual) assignment of IP addresses
is used is up to the administrator of the network. The administrator may opt
for DHCP since this can be a lot less work.
Making DHCP and DNS Work Together
Since under DHCP the IP addresses alter regularly, ever
time a machine is logged off and back on the network, these dynamically allocated
addresses are not placed in the DNS lookup tables. In order to make the two
systems work together a system called Dynamic DNS (DDNS) is used. DDNS allows
domain names to be linked to dynamically assigned IP addresses.
by
Matthew
Martin
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