DHCP and Network Settings

1
DHCP and Network Settings
Presentation written by Carol A. Hopkins.
Presented on March 10, 2008 for Seton Hall CSAS
4081, taught by Dr. Wachsmuth

• What is DHCP and its function, what is a Gateway
  and why do we need one, what is DNS?

2
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.

3
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.

4
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.

5
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.c
  om/topic/ip-address?cattechnology)
• 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
  ?cattechnology)
• 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)

6
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

7
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)

8
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)

9
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.dhcp-handbook.com/dhcp_f
  aq.html)

10
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)

11
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 RFCs (once
  source you can consider to look up RFCs on DHCP
  and DNS is http//www.bind9.net/rfc-dhcp)
• Related protocols BOOTP, Bootstrap Protocol. The
  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/lib
  rary/8e75e9f0-72e0-4b06-b6dd-abf88e652d3a1033.mspx
  ?mfrtrueInternet standards
• Whats an RFC? Its a Request for Comment.
  RFCs refer to a documented standard for new or
  modified internet or networking protocols. More
  info http//docs.info.apple.com/article.html?artn
  um24867

12
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).

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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_28telecommun
  ications29
• http//en.wikipedia.org/wiki/Gateway_28telecommun
  ications29

14
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/wik
  i/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/)