DHCP and Network Settings What is DHCP and its function, what is a Gateway and why do we need one, what is DNS? Presentation written by Carol A. Hopkins. Presented on March 10, 2008 for Seton Hall CSAS 4081, taught by Dr. Wachsmuth

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 , by which it can be located within a network. ( address?cat=technology) address?cat=technology The traditional IP address uses a 32-bit number that defines both the network and the host computer. ( 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. (

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. (

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) (

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 ( 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. ( handbook.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. (

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 (once source you can consider to look up RFC’s on DHCP and DNS is 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.BOOTP 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:

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 ( Gateway (album) 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.

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 ( into IP addresses ( ) and to control mail delivery ( 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 Happy Birthday to DNS. It is 25 years old this year. (