Transcript DHCP and Network Settings

Presentation written by Carol A. Hopkins.
Presented on March 10, 2008 for Seton Hall
CSAS 4081, taught by Dr. Wachsmuth
DHCP and Network Settings
What is DHCP and its function, what is a Gateway and why do we need one, what is DNS?
What is DHCP?



Decentralized Hospital Computer Program
DHCP is a communications protocol that stands for
DYNAMIC HOST CONFIGURATION PROTOCOL.
The DHCP server manages a pool of IP addresses and
information about client configuration parameters such
as the default gateway, the domain name, the DNS
servers, time servers, etc.
PURPOSE OF DHCP

DHCP automates the assignment of unique IP
addresses, subnet masks, default gateways, and other
IP parameters to individual computers and devices on
the network.

It also reduces the work necessary to administer a large
IP (Internet Protocol) network. DHCP lets a network
administrator supervise and distribute IP addresses from
a central point and automatically sends a new IP
address when a computer is plugged into a different
place in the network.
Without DHCP Servers ……





Network Administrators would be over-worked, and
underpaid.
The desktop client would be responsible for assigning a
proper IP address within the appropriate range.
Two different clients may end up claiming the same IP
number.
Desktop clients will need too much knowledge about IP
address ranges, etc. This for example could lead to
problems when the network ranges change.
Will make it difficult to move a computer from one
subnet to another.
What is an IP address?
IP stands for Internet Protocol.
 Definition of IP address: A computer's numeric
address, such as 149.150.51.69, by which it can
be located within a network.

(http://www.answers.com/topic/ip-
address?cat=technology)

The traditional IP address uses a 32-bit number
that defines both the network and the host
computer.
(http://www.answers.com/topic/ip-address?cat=technology)
Note: An IP address is not the same as a MAC address. A MAC address uniquely identifies a
computer that has an ethernet interface. Unlike DHCP it includes no indication of where your
computer is located. (http://www.dhcp-handbook.com/dhcp_faq.html)
What is an IP address, continued



IP addresses allow firewalls to block other networks based
upon their IP Address (black listing), or to allow from only
particular networks (white listing).
IP Version 4 addressing – uses 32 bit (4 byte) addresses,
which limits the address space to 2 to the 32 power. Many
are reserved for certain purposes.
IPv4 address are usually 4 numbers, ranging from 0-255,
separated by dots. Each is called an octet. In IPv4, Class A
networks (largest) are identified by the first octet which
ranges from 1 to 126. Class B networks are identified by
the first 2 octets, and range from 128 through 191. Class C
networks (the smallest) are identified by the first 3 octets,
the first of which ranges from 192 to 233.
Source: Wikipedia
Basic DHCP Protocol – 4 Phases
DISCOVER PHASE:




When a DHCP configured devices connect to the
network, the client sends a broadcast request (called a
DISCOVER or DHCPDISCOVER) looking for a DHCP
server to answer.
The router directs the DISCOVER packet to the correct
DHCP server.
The DHCP server receives the DISCOVER packet.
Based up on availability the server determines an
appropriate IP address to give to the client.
(http://kb.iu.edu/data/adov.html)
Basic DHCP Protocol, continued
OFFER PHASE:
The server temporarily reserves the IP address and
send back the client an OFFER (or DHCPOFFER)
packet with the address info.
The server also configures the clients DNS servers,
WINS servers, NTP servers, etc. (see below)
(http://kb.iu.edu/data/adov.html)
Basic DHCP Protocol, continued
REQUEST PHASE:
The client sends a REQUEST (DHCP REQUEST) packet,
letting the DHCP server know that it intends to use
that address.
ACKNOWLEDGEMENT PHASE:
The Server sends an ACK (or DHCPACK) packet
confirming client has been given a lease on the
address
(http://kb.iu.edu/data/adov.html)
A DHCP Lease is the amount of time a DHCP server grants the client permission to use a particular
IP address. The Administrator of the DHCP server can set this. (http://www.dhcphandbook.com/dhcp_faq.html)
DHCP – ways of allocating IP
addresses



Manual allocation: (static IP addresses): The server's
administrator creates a configuration for the server that
includes the MAC address and IP address of each DHCP
client that will be able to get an address.
Automatic allocation: the server's administrator creates a
configuration for the server that includes only IP
addresses, which it gives out to clients. An IP address,
once associated with a MAC address, is permanently
associated with it until the server's administrator
intervenes.
Dynamic allocation: like automatic allocation except that
the server will track leases and give IP addresses whose
lease has expired to other DHCP clients.
(http://www.dhcp-handbook.com/dhcp_faq.html)
DHCP – oddball facts

DHCP messages


Client to server: sent to UDP on port 67
Server to client: sent to UDP on port 68.

The for DHCP protocol can be found in RFC’s

Related protocols: BOOTP, Bootstrap Protocol. The
(once source you
can consider to look up RFC’s on DHCP and DNS is http://www.bind9.net/rfc-dhcp)
bootstrap protocol (BOOTP) is a host configuration protocol
developed before DHCP. DHCP improves on BOOTP and
resolves specific limitations BOOTP had as a host
configuration service. RFC 951 defines BOOTP.
http://technet2.microsoft.com/windowsserver/en/library/8e75e9f0-72e0-4b06-b6dd-abf88e652d3a1033.mspx?mfr=trueInternet standards

What’s an RFC? It’s a “Request for Comment”. RFC’s refer
to a documented standard for new or modified internet or
networking protocols. More info:
http://docs.info.apple.com/article.html?artnum=24867
What is a Gateway

In earlier times a gateway was earlier name for router.

The “default gateway” is a router, which is used to forward
traffic that is not addressed to a particular station within
the local network.
In your home, the default gateway directs Internet traffic from
the local network to the Cable/DSL modem, which connects to
your ISP (Internet Service Provider).
But, what is a Gateway besides a
router?

Gateway (album), an album by stoner metal band Bongzilla
(http://en.wikipedia.org/wiki/Gateway)
A gateway allows two different networks to connect and
understand each other, it is the node on a network that
interfaces with a another network that may use different
protocols.
In a big network, a computer server acting as a gateway
node is often also acting as a proxy server and a firewall
server. A gateway is often associated with:


a router, which knows where to direct a given packet of data that arrives at the gateway (such
as previously mentioned packets in DHCP protocol, such as DHCPDISCOVER, DHCPOFFER, etc),
and a switch, which furnishes the actual path in and out of the gateway for a given packet.
Gateways, also called protocol converters, can operate at
any layer of the OSI MODEL (Open Systems Interconnection
Basic Reference Model). The job of a gateway is much more
complex than that of a router or switch. Typically, a
gateway must convert one protocol stack into another.
http://searchnetworking.techtarget.com/sDefinition/0,,sid7_gci212176,00.html
http://en.wikipedia.org/wiki/Gateway_%28telecommunications%29
http://en.wikipedia.org/wiki/Gateway_%28telecommunications%29
What is DNS?
As previously mentioned, DNS information is also provided
by the DHCP server.
DNS Stands for DOMAIN NAME SYSTEM.
 DNS serves as the “phonebook” for the Internet by
translating computer host names (www.shu.edu) into IP
addresses (149.150.51.69) and to control mail delivery
(http://en.wikipedia.org/wiki/Domain_name_system)

Try typing Type ‘nslookup sciris.shu.edu’ (without the quotes!!) on
your Linux account.

There are websites you can go to translate server IP
addresses to DNS, and vice versa, such as
http://www.whois.net

Happy Birthday to DNS. It is 25 years old this year.
(http://www.dns.net/dnsrd/)