Specifications

Chapter 1 Understanding Networking and IP Addressing
Networking using IP
Niagara Release 2.3
Niagara Networking & Connectivity Guide Revised: May 22, 2002
1–27
Note One of the advantages of a HOSTS file is that it is not dependent on a server for name
resolution, as is required in the other resolution protocols.
The HOSTS file is always the first place a host looks for name resolution, and if it
finds an entry it uses it and does not check other name sources.
DNS The domain name system is the mechanism used by hosts to resolve names on the
Internet, and on some private networks as well. Hosts that participate in the DNS
system have names like “www.tridium.com”. This is referred to as the fully qualified
domain name. As with IP addresses, the name is broken down into two parts, the host
portion (in this example, “www”) and the domain name (tridium.com). Domain
names must also be globally unique so the data intended for that domain gets there
and not to some other address. To accomplish this, the domain names are controlled
by ICANN, and governed like the IP address system with authority granted to
accredited name registrars located throughout the world.
Name Servers
The first level of the domain naming system is organized into classes such as .info,
.com, and .org, and countries, such as .uk, and .bm. Top-level domains are then
divided into second-level domains such as va.us, co.uk, and tridium.com. A second
level domain can then be subdivided into a third level (such as bbc.co.uk) and so
forth. Domains can actually be subdivided into 127 levels, but it is rare to see a name
with more than 4 levels.
The servers that provide resolution for the naming system are also arranged
hierarchically. At the top, there are servers referred to as the root name servers. These
servers maintain a table of IP addresses of the first level servers (those serving .com,
.org, etc.) The first-level servers know the IP addresses of the name servers for the
second level, and the second level servers know the addresses for the third level
servers, and so forth. The name servers at the lowest level of the domain name (such
as bbc.co.uk) keep track of the IP addresses and names of all the hosts within the
domain. Most of the information tracked by these servers is maintained by manually
adding entries to the tables.
DNS is one of the largest and busiest distributed database systems in the world. It is
probably successful due to two factors: redundancy of servers and caching. At each
level there are multiple servers returning queries for their level. In addition, once a
name server learns the address of the server for a level (such as .com or co.uk) it
caches (stores) that information for faster lookup by another host and to reduce
additional queries.
How it Works from the Host
Many host operating systems (OSs) support DNS. Administrators configure the host
to look up entries in one or more DNS servers. The host can be configured manually,
or it can dynamically receive this setting with DHCP.