CompTIA Network+ Certification Study Guide part 35 pptx

Consider the following: You need to allow your clients to communicate with the DHCP server to get an address so they can participate on the network, accessing services and so on.. Becaus

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In a more technical definition, DHCP is a communications protocol that allows you to manage IP addressing usage centrally and to automate the assignment of logical addresses in an organization’s network Remember, each host on the network needs a unique IP address to be able to commu-nicate When an organization sets up its computer users with a connection

to the Internet, an IP address must be assigned to each machine Without DHCP, the IP address must be entered manually at each computer and, if computers move to another location in another part of the network, a new

IP address must be entered 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

DHCP uses the concept of a lease, or amount of time, that a given IP

address will be valid for a computer The lease time can vary depending on how long a user is likely to require the Internet connection at a particular location It’s especially useful in education and other environments where users change frequently Using very short leases, DHCP can dynamically reconfigure networks in which there are more computers than there are available IP addresses

DHCP supports static addresses for computers containing Web serv-ers that need a permanent IP address; you can make reservations for such addresses

DHCP is an extension of an earlier network IP management protocol, Bootstrap Protocol (BOOTP) DHCP is a more advanced protocol, but both configuration management protocols are commonly used and DHCP can handle BOOTP client requests Some organizations use both protocols, but understanding how and when to use them in the same organization is important Some operating systems, including Windows NT/2000, come with DHCP servers A DHCP or BOOTP client is a program that is located

in (and perhaps downloaded to) each computer so that it can be configured

DHCP Operations

DHCP was covered briefly earlier in the chapter DHCP is responsible for automatic and dynamic addressing of your network It has a lot of com-plexity to it as well For example, to get DHCP broadcasts to get to remote

Note

DHCP and its operation are thoroughly covered within the DHCP RFC www.rfc-editor org/rfc/rfc2131.txt

sites that are connected only by routers and T1 links, you would need to

configure those routers to pass the DHCP broadcast from the client to the

server; if the router is not configured to do so, then it will not pass This is

a common problem seen on the Network exam Consider the following:

You need to allow your clients to communicate with the DHCP server to get

an address so they can participate on the network, accessing services and

so on You have three clients on one remote subnet that cannot get a valid

IP address, but all other clients can This is a common issue Because all

other sites work just fine (eliminating the possibility that it could be a server

issues affecting all sites) the problem may be that the remote site’s router

is not configured to pass the broadcast from the client to the server, which

will then give that client a lease on an IP address so it can participate on the

network In this section we cover the basics of DHCP operations

As just mentioned, when a DHCP-based client is booted up, unless

already configured with an IP, the client attempts to communicate with a

DHCP server to get its TCP/IP configuration information The following is

a list of DHCP message types exchanged between client and server You will

not need to memorize these for the Network exam, although

understand-ing these messages simplifies the understandunderstand-ing of DHCP itself and better

prepares you for the exam

■

■ Dhcpdiscover The first time a DHCP client computer attempts

to start on the network, it requests IP address information from a

DHCP server by broadcasting a Dhcpdiscover packet The source

IP address in the packet is 0.0.0.0 because the client does not yet

have an IP address The attempt is sent out from the client on the

network and as long as the packet can get to the server, the request

process can be officially completed by the server

■

■ Dhcpoffer When the DHCP server receives the request, it selects

an unleased IP address from the range of available IP addresses and

offers it to the DHCP client The lease is generally configured as

part of a scope, as mentioned earlier The lease is good generally for

a week by default, although this can be changed In most cases, the

DHCP server also returns additional TCP/IP configuration

informa-tion, such as the subnet mask and default gateway in a Dhcpoffer

packet More than one DHCP server can respond with a Dhcpoffer

packet, and the client accepts the first Dhcpoffer it receives

■

■ Dhcprequest When the client receives the Dhcpoffer packet, it

responds by broadcasting a Dhcprequest packet that contains the

offered IP address

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■

■ Dhcpdecline A message from the DHCP client to the server

indi-cating that the offered configuration parameters are invalid

■

■ Dhcpack The DHCP server acknowledges the client’s Dhcprequest

for the IP address by sending a Dhcpack packet

■

■ Dhcpnack If the IP address cannot be used by the client because it

is no longer valid or is now used by another computer, the DHCP server will respond with a Dhcpnack packet

■

■ Dhcprelease A message from the DHCP client to the server that

releases the IP address and cancels any remaining lease

DhCP relay Agents

When the DHCP server receives the request from the DHCP client com-puter, it dynamically assigns an IP address to the requesting computer from the range of valid IP addresses contained within the DHCP scope The DHCP server allocates the IP address with a lease that defines how long the IP address can be used by the client computer The DHCP server can also establish other configuration parameters, such as subnet mask and Domain name system (DNS) and Windows Internet Name Service (WINS) server identification for the client computer DNS and WINS are both cov-ered within this chapter It’s important to remember that when configuring DHCP for clients, it’s not just an IP address that is delivered to the cli-ent, but many other parameters such as DNS server address, WINS server address, subnet mask, default gateway, and routing metrics, all of which are covered within this chapter

To get this information to the client so that it can be used, the client must

be able to contact the DHCP server As mentioned earlier, if it cannot, then you may have a router issue that prevents the broadcast request from getting through Understanding and configuring DHCP relay agents on a router is

a very important part of DHCP to consider as a network engineer TCP/IP networks are interconnected by routers that connect network segments (sub-nets) and pass IP packets between the subnets Because routers do not pass broadcasts by default, a configuration change must be added to the router

As mentioned earlier, one of the major components of the DHCP specifica-tion is the DHCP protocol for communicaspecifica-tions between DHCP servers and clients If this communication is disrupted or not allowed, DHCP will not function on your network

On the Network exam, you may come across a question or two that tests your knowledge of RFC 1542 and broadcast-based communications when working with an RFC 1542-compliant router A DHCP server can

only provide IP addresses to clients in multiple subnets, if the router that

connects the subnets is an RFC 1542-compliant router The configuration is

commonly called an IP helper address in Cisco Systems-based routers If the

router cannot function as a relay agent, each subnet that has DHCP clients

requires a DHCP server

A relay agent is a program used to pass specific types of IP packets between

subnets A DHCP/BOOTP relay agent is simply a hardware or software

pro-gram that can pass DHCP/BOOTP messages (packets) from one subnet to

another subnet according to the RFC 1542 specification

Now that you understand the basics of network protocols such as IPX/

SPX, AppleTalk, and TCP/IP, let’s continue learning about the TCP/IP suite’s

other functionalities, services, applications, and protocols In the next

sec-tion, we will briefly cover the use of multicasting and the TCP/IP used to

provide it

MulTICAST, BroAdCAST, ANd uNICAST

With the continuously expanding use of networks, more and more people

are deciding that one-to-one networking is not enough anymore The need

to have one-to-many networks has become more important This is true

for large corporations that benefit from e-mail, file sharing, and mirrored

servers in two different cities (or countries) New technologies are developed

every day

Multicasting can reduce travel expenses while maximizing benefits

Imagine the cost of sending several employees halfway around the world for

a conference that lasts less than a day Not only would you incur the cost of

travel, but also the cost of the employees’ time as they travel

A better solution in this case would be to videoconference (which is a

very popular and always-developing technology), which allows viewing a

Note

BOOTP is described in RFC 951 and RFC 1084 and is used for booting diskless

nodes Updated in RFC 1395 and RFC 1497 and superseded by DHCP, BOOTP is still

supported for legacy applications on most, if not all DHCP server implementations The

way it works is that when the client is ready to boot up on the network, it sends out a

broadcast message requesting information and waits for a reply The client only has to

know its own hardware (Media Access Control, MAC) address With this information, the

BOOTP server will respond with an IP address.

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presentation in one window while watching the speaker in another Ques-tions can be typed while the presentation is in progress, and prioritized for answering at the end of the conference These are just a few of the features that can be provided by multicasting Other benefits can include interactive distance learning and corporate announcement transmissions

Multicasting benefits are not limited to video/audio needs Multicast can

be used to push updates to multiple hosts simultaneously, thus reducing the effort and time involved in doing one update at a time Multicasting can also push computer operating system images to their hosts

The possibilities seem endless and are rapidly growing The following sections cover the basics of multicasting and how the multicasting address-ing scheme is laid out

understanding the Basics of Multicasting

For the Network exam, you will neither need to understand the dozens of commands that you can program into a router to enable and control multi-casting, nor will you need to know the exact detailed operation of how mul-ticasting protocols such as Internet Group Management Protocol (IGMP) work However, you do need to know about multicasting fundamentals for the exam You have already learned about Class D addressing space, which

is where multicasting was originally mentioned Why is there so much con-cern about it? Well, for one, because the use of it is growing, it must mean that the pressure placed on networks today is warranting its use Bandwidth utilization is the first thing most network technicians and administrators think about when discussing streaming video and other live information feeds to an individual’s PC or across a WAN link that may not have the bandwidth to accommodate it To simply increase your bandwidth because

of a single application’s requirements could be expensive when dealing with telecommunications providers

To understand multicast traffic completely, we have to discuss the other types of traffic It is important to understand the differences between uni-cast, broaduni-cast, and multicast traffic Multicasting is UDP-based Although UDP is not a great example of reliability, it makes more sense for multicast-ing than TCP For starters, havmulticast-ing a multitude of hosts acknowledge receipt

of a multicast packet stream would be counterproductive Additionally, UDP has lower overhead, which provides the speed necessary to support the traffic needs of multicasting

Multicast addresses cannot be used as source addresses for any traffic Although multicast addresses can be associated with particular interfaces on particular devices (such as 224.0.0.5 for Open Shortest Path First- enabled

(OSPF) interfaces on a router), traffic cannot be sourced from a multicast

address because it does not identify a specific host; rather, a multicast address

identifies a group of hosts sharing the same address

Multicast addresses are not assigned to a device; rather, a device

pro-ceeds to listen for and receive traffic destined to a multicast group that

it has joined by some process For example, routers can join the OSPF

multicast group on their network by having OSPF configured, and

hav-ing interfaces configured to participate in OSPF routhav-ing In this case, it

means that the router will receive traffic destined to multicast IP addresses

reserved for OSPF routing Hosts can opt to join a multicast group by

hav-ing certain applications (such as videoconferenchav-ing software) installed and

configured

Note

Remember, the Network+ exam does not dig as deeply into these concepts as this

chapter does (such as our last discussion on OSPF) You need to remember facts, such

as which protocols are used, which IP address class is used, which IP range is within

that class, as well as being able to single out any wrong answers that may be placed

in the question as a distracter Knowing this other information is only going to help you

understand what you are memorizing Understanding multicasting is very important as

a network engineer, especially if you are working with videoconferencing or any of the

many other applications that use multicasting as an underlying technology.

As mentioned earlier, IGMP allows host computers on the Internet to

participate in IP multicasting A multicast address identifies a transmission

session instead of a particular physical destination This allows for sending a

message to a large number of recipients without the necessity for the source

computer to know the addresses of all the recipients The network routers

translate the multicast address into host addresses The protocol used to

facilitate this is IGMP IGMP was originally defined in RFC 1112

Exten-sions have been developed and are included in IGMP version 2, addressed

in RFC 2236

A computer uses IGMP to report its multicast group memberships to

multicast routers IGMPv2 allows group membership terminations to be

reported promptly to the routing protocol IGMP is required to be used in

host computers that wish to participate in multicasting IGMPv3 is also

available for use Knowing all the version types is not necessary for the

Network exam, but it’s important to know if you need to use IGMP, as

some versions have (obviously) more functionality, enhancements, and

security than others

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unicast Traffic

What is most commonly seen (and wanted on your network) is what is called unicast traffic Unicast is the transmission of data from one host to another, one host at a time This is a one-to-one session between one host and another, such as a client and server arrangement Unicast can be used

to support multiple sessions (that is, multicasting) by establishing multiple one-to-one communications to transport the same data stream to multiple hosts An example of this is shown in Figure 7.5

If the session is required by multiple hosts, a one-to-one connection is established, with the same data transmitted repeatedly to each host This form of transmission will not transmit to every computer on a network; however, multiple requests for the same conference or data would cause that data to be pushed across the network media at the same time Thus, as shown in Figure 7.5, a video feed of 1.5 Mbps unicasted to 10 computers on

a network requires 15 Mbps of bandwidth Although this might not seem significant, it can degrade network performance as the feed size and quantity increase

The toll of network usage is realized on the network equipment traversed from source to destination for the video feed All of the routers and switches will have a considerable amount of data traffic to process

FIGurE 7.5 Unicast Network Video Feed Example.

Broadcast Traffic

Broadcast is another option that can be used for transmitting data to a large

number of host systems simultaneously Broadcasts can consume a

signifi-cant amount of bandwidth; connections are based on a one-to-all method

transmission This can be seen when using the NetBIOS and ARP

proto-cols, as well as many others Any hosts on a network where a broadcast is

generated will process that broadcast (at least far enough to know it is not

intended for that system)

The broadcast traffic is sent to all computer systems that can be reached

on the network This process launches the 1.5 Mbps video stream to all the

interfaces possible, thus not creating the intense bandwidth consumption of

a unicast

The problem is that each host receiving the broadcast has to process

the 1.5 Mbps data stream continuously until it is finished If the

receiv-ing host does not want the broadcast traffic, valuable resources of the

host will still accept the datagram and then determine what to do with

it – accept it or reject it Because this is also a video feed, this large piece

of data has to be processed, which can take a considerable toll on the host

system

Another disadvantage of using the broadcast transmission for video

feeds is the network architecture On a small network with no routers, this

may be a desirable option On larger networks, or if there are any routers

in the path to a host, the default action is to filter (block) the broadcast,

meaning that broadcasts must be explicitly allowed to traverse the path to

the host

Multicast Traffic

Obviously, neither unicast nor broadcast is optimized to handle traffic

des-tined for multiple hosts, especially if those hosts are logically assigned to a

specific group Multicasting and the protocols discussed address this need

Multicast traffic establishes a one-to-many type of transmission This allows

the data traffic to only be sent to those who specifically requested the

infor-mation, and only sends one stream of traffic to each requesting broadcast

domain

Multicast (RFC 1112) is a technology used to address multiple hosts as a

group A source host multicasts to a group of hosts by sending an IP packet

to a special IP address associated with that group The IP address that defines

a multicast group is a Class D address (224.0.0.0 to 239.255.255.255), with

unique groups allocated their own IP address in that range This allows

multiple multicast groups to be defined at the same time with different

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IP addresses Multicasting sends the data stream only to the group of hosts that specifically want it All other hosts ignore and do not process the mul-ticast traffic

Multicasting differs from broadcasting because multicasting sends traffic

to a group of hosts, not to all hosts on a network Hosts that are not part of the group will not process the multicast packet because it is not addressed

to them

As mentioned earlier in the section, a typical multicast application is videoconferencing Not all network users want or need to participate in a videoconference; only those users that need to will join the multicast group

to receive the video feed

The advantage of multicasting becomes apparent when you consider that using unicast addresses would result in an individual video feedback to each receiver More users and demand mean more bandwidth used By using mul-ticasting, only one channel is used, regardless of the number of users: 1000 users only require one channel Multicast traffic is bidirectional: a host can receive or send multicast packets

As mentioned briefly before, it’s important to understand the need for the group If only one data stream is being transmitted, how can all of the requesting systems receive the data? Multicasting uses IP addresses to estab-lish multicast groups, which host systems can join to receive multicast data The multicast data is sent to the group IP address and all listed group mem-bers receive the traffic

Multicast Ip Address designations

Class D IP addresses comprise the whole range of multicast addresses, with a range of 224.0.0.0 through 239.255.255.255 Multicast IP addresses are eas-ily recognized by their binary numeration, as their high-end bits are always

1110 For instance 11100000 is equal to 224 and 11101111 is equal to 239 These first 4 bits account for a portion of the IP address; the remaining 28 bits are used for multicast group identification Two types of multicast IP

addresses are used: dynamic and static.

Transient (dynamic) addresses are used for the duration of the ses-sion and are relinquished when no longer needed Dynamic multicast IP addressing allows applications to acquire an IP address for the length of the multicast transmission This IP address allocation has a certain expiration time and must be considered by the application requesting the address to retain functionality For example, a transient address is used to multicast

a videoconference of an event After the event is finished, the transient address can be reused Transient addresses must be coordinated to ensure

that two people or organizations do not use the same transient address for

different needs

Static multicast IP addresses are a group of IP addresses, ranging from

224.0.0.0 to 224.255.255.255, that have been specifically assigned by the

IANA The permanent addresses are defined in the protocol itself, such as

the all-hosts (224.0.0.1), all-routers (224.0.0.2), or RIPv2 group (224.0.0.9)

addresses Permanent addresses can also be assigned by the IANA for other

protocols or uses These addresses are reserved for particular purposes and

are referred to as well-known addresses For a complete listing of statically

assigned Class D IP address, see

addresses

All reserved static Class D addresses that are used for multicast

man-agement and multicast data are never forwarded to these addresses Static

addresses such as 224.0.0.2 include all multicast-enabled router interfaces

Multicast-enabled routers automatically join this “all routers” group upon

initialization In turn, all multicast-enabled hosts must join the all-host

sys-tems group 224.0.0.1 Others become active upon activation or

configura-tion of some features such as OPSF on a router

uNdErSTANdING BASIC Ip rouTING

In this section, we’re going to explore how data is routed on a network using

the IP protocol We’ll begin by discussing how names and addresses are

resolved Then, we’ll look at how packets of data are sent from one network

to another to understand the process of basic IP routing Understanding how

routing works will help you to understand the concepts behind routing

pro-tocols In this section, we will thoroughly cover how data is transmitted on

a TCP/IP network This knowledge is easily converted to other suites (such

as IPX/SPX) Less commonly used protocols are not covered as thoroughly

on the exam, hence we are focused on TCP/IP in this section (and chapter)

However, you will need to know how to use protocols and services within

the other suites to be able to function in a production environment that may

not solely rely on TCP/IP for communication Consider this as you wrap up

your studies for this exam Think about moving on to other protocols later

and dig into them at a much more involved level

Test day Tip

Memorize the Class D range, not the specific assignments for the exam The specific

assignments are for your own knowledge.