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Network+ TechNotes
Network Services

DHCP/bootp

The Dynamic Host Configuration Protocol (DHCP) is a service used in TCP/IP networks to assign automatic IP addressing configuration to network nodes. DHCP consist of a server part and a client part. The server is a service typically installed on a server or running on a router for example. The client part is installed on clients, servers, and other devices, and requests an IP address configuration from a DHCP server. DHCP significantly simplifies administration and ensures every host will use a unique IP address as is required in an IP network.

When a DHCP client boots for the first time, it will use the bootp protocol to request a DHCP server to issue an IP address. This is called the lease process and goes as follows:

1. The client sends out a DHCPDiscover broadcast message to request IP addressing information from a DHCP server.
2. One or more DHCP servers respond with a DHCPOffer, containing an IP address and other IP addressing info such as subnet mask and default gateway. The first DHCPoffer received by the client is will be accepted, others will be ignored.
3. The client responds with a DHCPRequest, a broadcast message containing the IP addressing information again to make sure it is still available and can be used.
4. If the address is still available, the DHCP server responds with a DHCPAck (Acknowledge) and the optional configuration, such as DNS and WINS servers. Once the client receives the acknowledgement, it will start using the new IP addressing configuration. Or, the DHCP server responds with a DHCPNak (Negative Acknowledge) when the IP address is no longer available, which forces the client to start the lease process all over again.

The IP addresses issued by the DHCP server are valid for a configurable amount of time, called the lease period. When 50% of the lease period has expired, the client will try to renew the lease for the same IP address. If this fails, the client will try again at 87.5% of the lease period. When a DHCP client is not able to locate a DHCP server, during the initial configuration or during the lease renewal attempt, the client will be configured with an IP address of 0.0.0.0. In case the client uses Automatic Private IP Addressing (APIPA), it will be configured with an IP address from the network 169.254.0.0, with the subnet mask 255.255.0.0.

DHCP servers listen to incoming messages at UDP port 67, and clients listen at UDP port 68. Routers typically do not forward UDP broadcasts, hence every subnet requires its own DHCP server. To overcome this limitation, a router can be configured to forward UDP port 67 and 68 broadcasts, or a DHCP relay agent can be installed in subnets without DHCP servers. The DHCP relay agent can be either a client or a server that picks up DHCP broadcasts and forwards them to a DHCP server in another subnet. That DHCP server responds to the DHCP relay agent, which in turn forwards the information to the DHCP client that sent the original broadcast. In other words, the DHCP relay agent acts as an intermediate between a DHCP client in one subnet and a DHCP server in another subnet.

Besides the IP address and the subnet mask, other IP addressing information is also typically issued by a DHCP server. These parameters include:

- Default gateway address
- DNS server addresses
- WINS server addresses

Besides for DHCP messages, the Bootp protocol is also used for bootstrapping. Bootstrapping allows a diskless client to boot from the network by loading the operating system from a central server.

Name Resolution

Compared to TCP/IP networks, most telephone systems are rather dumb. In general, when you want to call someone, you have to dial an x-digit number. In TCP/IP networks you can contact an intended communication partner by using a name instead of having to know a numeric address for every computer you want to contact. For this to work, there has to be some naming system that can resolve names to IP addresses. The two main services taking care of this are DNS and WINS.

DNS

Today's most common naming system in corporate IP networks and ‘the’ naming system on the Internet is the Domain Naming System (DNS). The primary function of DNS is to resolve host names to IP addresses, and vice versa. A DNS server maintains a hierarchical directory, or a portion of it, in a database with zone for each domain. Records are created in a zone to map host names of individual network resources to their IP addresses. Following are some common example of resource records:

A

This is the hostb part in the FQDN below this table and maps a host name to an IP address.

CNAME

This is an alias for an A record, for example the www part in www.tedomain.net could actually be an alias for host11.tedomain.net. And mail.tedomain.net and ftp.tedomain.net could be the same host as well.

MX

This name maps to the IP address of an SMTP server to which email for this domain should be send. For example: mail.tedomain.net.

PTR

A pointer record is the opposite of an A record. It maps an IP address to a hostname instead of vice versa. This allows DNS clients to resolve an IP address to host name.

A host name is actually a part of a 'larger' name, called a Fully Qualified Domain Name (FQDN). Here's an example of an FQDN:

hostb.tedomain.net

This name consists of three parts read from right to left:
net is the top-level domain
tedomain is the second-level domain
hostb is the host name.

There is actually another level on top of the top-level domain, which is called the root and is sometimes actually represented in an FQDN, right from the top-level domain, as a dot.

When a client wants to communicate with another host in the network by using a host name, it connects to UDP port 53 on the DNS server and requests the IP address of the target host. If the zone for the domain of the hostname is located on the DNS server, it will reply with the IP address. If the zone is located on another DNS server, on the Internet for example, the DNS server can forward the request and act as an intermediate between the client requesting the IP address and the DNS server hosting the database with the actual record.

The HOSTS file is the local static equivalent and predecessor of DNS. It is a text file that contains IP address to host name mappings. It originated on UNIX but can be found on Windows OS clients and servers as well. Following is example content of a HOSTS file:

102.54.94.97 server1.tedomain.net # source server
38.25.63.10 server2.tedomain.com # x client host
127.0.0.1 localhos
t

On Windows NT-based systems such as Windows XP and 2000, the HOSTS file is located in the C:\WINDOWS\system32\drivers\etc folder. On Windows 9x the file can be found in the C:\WINDOWS\ folder.

WINS

The Windows Internet Naming System (WINS) was the primary naming system in Microsoft networks. Since the introduction of Windows 2000, DNS took over the role of WINS, but the latter is still available in Windows products to maintain compatibility with older systems. WINS maps NETBIOS names to IP addresses, and was used heavily in Windows NT 4 networks. Read the NETBEUI/NETBIOS TechNotes for more information about NETBIOS names and the difference with host names.

When a station without access to a WINS server uses a NETBIOS name to contact another station, the station will send a broadcast to discover the name of its intended communication partner. When that station receives the broadcast message, it will respond with its IP address so an IP connection can be established. To reduce the amount of broadcasts on the network, clients can be configured to consult a WINS server for NETBIOS to IP address mappings. All WINS clients register their name at the WINS server at startup to populate the WINS database on the WINS server. When they need to resolve a NETBIOS name to an IP address, they contact the WINS server using a direct unicast connection instead of generating broadcasts.

Besides the difference that WINS is used for NETBIOS names to IP address name resolution and DNS for host name to IP address name resolution, another main difference between DNS and WINS used to be that the WINS database is dynamic and DNS was static. WINS clients register and update their own records, although you can also add static entries to a WINS database. Most of the DNS servers on the Internet are still static, but modern implementation of DNS, such as those in Windows 2000 and 2003 can also be dynamic.

The LMHOSTS file is the local, static equivalent and predecessor of WINS. It is a text file that contains IP address to NetBIOS name mappings. It originated on Lan Manager (Microsoft's operating system before Windows) but used to be commonly configured on Windows OS clients and servers as well. Following is a sample entry of a LMHOSTS file:

102.54.94.97 teserver1 #PRE #DOM:tedomain

On Windows NT-based systems such as Windows XP and 2000, the file is located in the C:\WINDOWS\system32\drivers\etc folder. On Windows 9x, the file can be found in the C:\WINDOWS\ folder. Note that the file is called lmhosts.sam by default, you will need to create a new file or rename the sample file (thus remove the .sam extension) before you can use it.

SNMP

The Simple Network Management Protocol (SNMP) is an application layer protocol that is primarily used to monitor, and gather information about, network systems and devices. An SNMP agent is installed on a managed device to send SNMP information to a central Network Management System (NMS). On the NMS, the information is stored in a Management Information Base (MIB), which can be used to produce graphs, reports, baselines and other useful overviews of the network.

The following are 3 of the basic commands supported by SNMP:

Read

A read command can be sent to an agent to request information about a managed device.

Trap

Trap messages are sent from the agent to an NMS when a certain event occurs. E.g. when a service stops or a network interface goes down.

Write

Besides passively monitoring and gathering information, SNMP can also be used to 'manage' a network by configuring managed devices using a Write command.

SNMP agents listen and respond to UDP port 161, trap messages are send to UDP port 162. When an agent is not able to communicate with an NMS in another network, verify that these ports are not blocked on an intermediate router or firewall. Besides operating over UDP and IP, SNMP can also be used in IPX and AppleTalk networks.

Network Attached Storage (NAS)

(Note: Although NAS is no longer listed in the Network+ exam objectives, it’s covered here because some related protocols are listed in the exam objectives. These remote file access protocols will be covered in more detail in another section in these TechNotes regarding operating specific networking.)

Network Attached Storage ( NAS) in its simplest form is a file server that runs on a dedicated device directly connected to the network. Usually a box containing several hard disks combined in a RAID set, it is directly attached to the network through connections ranging from 10Mbps to 1Gbps and faster. Many NAS devices are based on Linux or UNIX derivatives and are usually easily installed, configured, and managed using a web browser. NAS can communicate with the network using TCP/IP, IPX/SPX, NetBEUI, or AppleTalk even. The primary advantage of this wide variety of supported protocols is that Windows, UNIX/Linux, Mac OS, and Novell clients can all use the same storage and access and share the same data.

These different operating systems each use one or more remote file access protocols to access data on a NAS device. Windows systems access files using either Server Messenger Block (SMB) or Common Internet File System (CIFS). Unix/Linux systems use the Network File System (NFS) . Novell systems use the Netware Core Protocol (NCP). And Apple systems use AppleShare or the Apple Filing Protocol (AFP). Additionally, most NAS devices also support file access through HTTP and FTP.

Do not confuse NAS with Storage Area Network (SAN). SAN is not a just a device, but refers to a complete network configuration where servers use central storage connected through fiber optic cabling or SCSI. Instead of being an autonomous device, the file system is dictated by the operating system running on the servers. SAN is commonly used in combination with clusters.



 
Current related exam objectives for the Network+ exam:

2.13 Identify the purpose of network services and protocols, for example:
- DNS (Domain Name Service)
- WINS (Windows Internet Name Service)
- SNMP (Simple Network Management Protocol)

4.6 Given a scenario, determine the impact of modifying, adding or removing network services for network resources and users. For example:
- DHCP (Dynamic Host Configuration Protocol)
- DNS (Domain Name Service)
- WINS (Windows Internet Name Service)



Click here for the complete list of exam objectives.

Discuss this TechNote here Author: Johan Hiemstra




 

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