Bridges, Switches,DNS,Ipv6

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    4.7 Interconnection of LANs - Bridges

    There are several ways of interconnecting networks. When two or more networks

    interconnected at the physical layer, the type of device is called a repeater. When two or more

    networks are interconnected at the MAC or data link layer, the device is called a bridge. When

    two or more networks are interconnected at the network layer, the device is called a router.

    Interconnection at higher layers is done less frequently. The device that interconnects networks

    at higher level is usually called a gateway. Gateways usually perform some protocol conversion

    and security functions. In this section we will focus on bridges as they are used to interconnect

    LANs at MAC layer.

    Bridges

    When range extension is the only problem, repeaters may solve the problem as long as the

    maximum distance between two stations is not exceeded. Local area networks (LANs) that

    involve sharing of media, such as Ethernet and Token ring, can only handle up to some

    maximum level of traffic. As the number of stations in the LAN increases, or as the traffic

    generated per station increases, amount of activity in the LAN medium increases until it reaches

    a saturation point. A typical approach is to segment the user group into two or, more LANs and

    to use bridges to interconnect the LANs to form a bridged LAN or an extended LAN.

    At times it becomes essential to have multiple LANs. There are several reasons for the use of

    multiple LANs interconnected:

    Geography. Clearly, two separate LANs are needed to support devices clustered in two

    geographically distant locations. Even in the case of two buildings separated by a highway, it

    may be far easier to use a microwave bridge link than to attempt to string coaxial cable between

    the two buildings.

    Performance. When range extension is the only problem, repeaters may solve the problem as

    long as the maximum distance between two stations is not exceeded. Local area networks

    (LANs) that involve sharing of media, such as Ethernet and Token ring, can only handle up to

    some maximum level of traffic. As the number of stations in the LAN increases, or as the traffic

    generated per station increases, amount of activity in the LAN medium increases until it reaches

    a saturation point. In general, performance on a LAN or MAN declines with an increase in the

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    number of devices or with the length of the medium. A typical approach is to segment the user

    group into two or, more LANs and to use bridges to interconnect the LANs. LANs will often

    give improved performance if devices can be clustered so that intra-network traffic significantly

    exceeds inter-network traffic.

    Reliability. The danger in connecting all data processing devices in an organization to one

    network is that a fault on the network or even a fault in a single device may disable

    communication for all devices in the network. The network may be required to be partitioned

    into self-contained units.

    Security. LANs originally assume an element of trust between the users in the LAN. As LAN

    grows, this assumption breaks down, and security concerns become prominent. The fact that

    most LANs are broadcast in nature implies that eavesdropping can be done easily. This behavior

    opens the door for the various security threats. Also it is desirable to keep different types of

    traffic (e.g., accounting, personnel, strategic planning) that have different security needs on

    physically separate media. At the same time, the different types of users with different levels of

    security need to communicate through controlled and monitored mechanisms.

    The establishment of multiple LANs and the filtering mechanism at the bridges will improve

    security of communications.

    Figure 4.16 Bridge interconnecting two LANs

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    Functions of a Bridge

    Figure 4.16 illustrates the operation of a bridge between two LANs, A and B. The bridgeperforms the following functions:

    Reads all frames transmitted on A, and accepts those addressed to stations on B.

    Using the medium access control protocol forB, retransmits the frames onto B.

    Does the same for B-to-A traffic.

    Several design aspects of a bridge are worth highlighting:

    1. The bridge makes no modification to the content of the frames it receives.

    2. The bridge should contain enough buffer space to meet peak demands. Over a short period of

    time, frames may arrive faster than they can be retransmitted.

    3. The bridge must contain addressing and routing intelligence. At a minimum, the bridge must

    know which addresses are on each network in order to know which frames to pass. Further, there

    may be more than two LANs interconnected by a number of bridges. In that case, a frame may

    have to be routed through several bridges in its journey from source to destination.

    4. A bridge may connect more than two LANs. The bridge provides an extension to the LAN thatrequires no modification to the communications software in the stations attached to the LANs. It

    appears to all stations on the two (or more) LANs that there is a single LAN on which each

    station has a unique address. The station uses that unique address and need not explicitly

    discriminate between stations on the same LAN and stations on other LANs; the bridge takes

    care of that.

    The description above has applied to the simplest sort of bridge. More sophisticated bridges can

    be used in more complex collections of LANs. These constructions would include additional

    functions, such as,

    Each bridge can maintain status information on other bridges, together with the cost and

    number of bridge-to-bridge hops required to reach each network. This information may be

    updated by periodic exchanges of information among bridges; this allows the bridges to perform

    a dynamic routing function.

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    A control mechanism can manage frame buffers in each bridge to overcome congestion. Under

    saturation conditions, the bridge can give precedence to en route packets over new packets just

    entering the internet from an attached LAN, thus preserving the investment in line bandwidth

    and processing time already made in the en route frame.

    Bridge Protocol Architecture

    The IEEE 802.1D specification defines the protocol architecture for MAC bridges. In addition,

    the standard suggests formats for a globally administered set of MAC station addresses across

    multiple homogeneous LANs. In this subsection, we examine the protocol architecture of these

    bridges.

    Within the 802 architecture, the endpoint or station address is designated at the MAC level.

    Thus, it is at the MAC level that a bridge can function. Figure 4.17 shows the simplest case,

    which consists of two LANs connected by a single bridge.

    Figure 4.17 Operation of a LAN Bridge from 802.3 to 802.4

    The LANs employ the same MAC and LLC protocols. The bridge operates as previously

    described. A MAC frame whose destination is not on the immediate LAN is captured by the

    bridge, buffered briefly, and then transmitted on the other LAN. As far as the LLC layer is

    concerned, there is a dialogue between peer LLC entities in the two endpoint stations. The bridge

    need not contain an LLC layer, as it is merely serving to relay the MAC frames.

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    2.8 Circuit Switching

    A network is frequently represented as a cloud that connects multiple users as shown in Figure

    2.28a. A circuit-switched network is a generalization of a physical cable in the sense that it

    provides connectivity that allows information to flow between inputs and outputs to the network.

    Unlike a cable, however, a network is geographically distributed and consists of a graph of

    transmission lines (that is, links) interconnected by switches (nodes).

    Figure 2.28 Network consists of links and switches

    As shown in Figure 2.28b, the function of a circuitswitch is to transfer the signal that arrives at

    a given input to an appropriate output. The interconnection of a sequence of transmission links

    and circuit switches enables the flow of information between inputs and outputs in the network.

    2.8.1 Space-Division Switches

    Space-division switchesprovide aseparate physical connectionbetween inputs and outputs so

    the different signals are separated in space. Figure 2.29 shows the crossbar switch, which is an

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    example of this type of switch. The crossbar switch consists of an NxNarray ofcrosspoints that

    can connect any input to any available output. When a request comes in from an incoming line

    for an outgoing line, the corresponding crosspoint is closed to enable information to flow from

    the input to the output. The crossbar switch is said to be nonblocking; in other words,

    connection requests are never denied because of lack of connectivity resources, that is,

    crosspoints. Connection requests are denied only when the requested outgoing line is already

    engaged in another connection.

    Figure 2.29 Crossbar Switch

    The complexity of the crossbar switch as measured by the number of cross-points is N

    2

    . Thisnumber grows quickly with the number of input and output ports. Thus a 1000-input-by-1000-

    output switch requires 106crosspoints, and a 100,000 by 100,000 switch requires 10

    10crosspoints.

    In the next section we show how the number of crosspoints can be reduced by using multistage

    switches.

    Multistage Switches

    Figure 2.30 shows a multistageswitch that consists of three stages of smaller space-divisionswitches. TheNinputs are grouped intoN/n groups ofn input lines. Each group ofn input lines

    enters a small switch in the first stage that consists of an n xn array of crosspoints. Each input

    switch has one line connecting it to each of k intermediate stage N/n x N/n switches. Each

    intermediate switch in turn has one line connecting it to each of the N/n switches in the third

    stage. The latter switches are k xn. In effect each set ofn input linesshares kpossiblepaths

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    to any one of theswitches at the laststage, that is, the first path goes through the first

    intermediate switch, the second path goes through the second intermediate switch, and so on.

    The resulting multistage switch is not necessarily nonblocking. For example, if k < n, then as

    soon as a switch in the first stage has kconnections, all other connections will be blocked.

    Figure 2.30 Multistage switch

    The number of crosspoints required in a three-stage switch is the sum of the following

    components:

    N/n input switches x nkcrosspoints/input switch. kintermediate switches x (N/n)2 crosspoints/intermediate switch.

    N/n output switches x nkcrosspoints/output switch.

    In this case the total number of crosspoints is

    2Nk+ k(N/n)2.

    The number of crosspoints required to make the switch nonblocking is

    2N(2n1) + (2n-1)(N/n)2

    The number of crosspoints can be minimized through the choice of group size n. By

    differentiating the above expression with respect to n, we find that the number of crosspoints is

    minimized ifn ~ = (N/2)1/2

    . The minimum number of crosspoints is then 4N((2N)1/2

    - 1). We

    then see that the minimum number of crosspoints grows at a rate proportional to N1.5

    which is

    less than theN2

    growth rate of a crossbar switch.

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    2.8.2 Time-Division Switches

    In the previous section, we explained how Time Division Multiplexing (TDM) could replace

    multiple physical lines by a single high-speed line. In TDM a slot within a frame corresponds to

    a single connection. The time-division switch uses time-slotinterchange (TSI) technique to do

    the switching operation. It basically switches the time slots of the input frame in the output

    frame.

    Figure 2.31 Time slot interchange TSI technique in Time division switch

    Consider users A & B who wishes to talk to each other. Suppose User A is assigned slot 3 and

    user B is assigned slot 5 then the time division switch will take the data information in the slot 3

    in the input frame and puts it into the slot 5 in the output frame. In effect information bits in the

    slot 3 which came from user A is going into the slot allocated to the user B in the output frame.

    This results in the one way connection from A to B. Similarly the time division switch put the

    information in the slot 5 in the input frame to the slot 3 in the output frame resulting in

    connection from B to A.

    The development of the TSI technique was crucial in completing the digitization of the

    telephone network. Starting in 1961 digital transmission techniques were introduced in the trunks

    that interconnected telephone central offices. Initially, at each office the digital streams would be

    converted back to analog form and switched by using space switches of the type discussed

    above. The introduction of TSI in digital time-division switches led to significant reductions in

    cost and to improvements in performance by obviating the need to convert back to analog form.

    Most modern telephone backbone networks are now entirely digital in terms of transmission and

    switching.

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    Circuit Switching

    In a circuit-switched network, a dedicated communication path is established between two

    stations through the nodes of the network. That path is a connected sequence of physical links

    between nodes. On each link, a logical channel is dedicatedto the connection. Data generated by

    the source station are transmitted along the dedicated path as rapidly as possible. At each node,

    incoming data are routed or switched to the appropriate outgoing channel without delay. The

    most common example of circuit switching is the telephone network.

    Packet Switching

    A quite different approach is used in a packet-switched network. In this case, it is not necessary

    to dedicate transmission capacity along a path through the network. Rather, data are sent out in a

    sequence of small chunks, called packets. Each packet is passed through the network from node

    to node along some path leading from source to destination. At each node, the entire packet is

    received, stored briefly, and then transmitted to the next node. Packet-switched networks are

    commonly used for terminal-to-computer and computer-to-computer communications.

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    Domain Name Servers

    Internet works on the numbering system. These numbers are called IP. When we connect

    to the net we have seen a set of 4 numbers being dialed i.e. for each address on the Internet there

    is a unique set of these 32 bit numbers. Domain Name Servers are the servers, which maintain adistributed list of all domains against Internet Protocol address.

    Earlier to Domain Name Servers there was a system of having a host table maintained by

    SRI-NIC. It was updated twice a week to include new sites. System would download the copy of

    this table through FTP.

    There are two types of servers as below:

    1. Resolver

    2. DNS

    There are a number of servers, which maintain the addresses of sites. When browser

    needs the address of any site, resolver queries the nearest name server, replies immediately if it

    knows the answer or it asks another server. Thus every server has two roles to play:

    1. As a server for name server.

    2. Super server to extend functionality.

    All web sites are arranged in 7 branches namely arpa, com, edu, net, gov, mil, org.

    Following this are 236 country name abbreviations like .in for India. This helps to locate the

    site easily. The IP addresses of name servers at each of the domain name tags are maintained by

    10 root servers.

    When a DNS fields a query that it cannot answer

    1. It sends a query to root server

    2. Root server says it does not know but a machine at say 195.95.251.10 might know

    3. DNS sends a query to the above machine

    4. Server at 195.95.251.10 knows the answer

    5. DNS returns answer to your PC.

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    Special features of DNS

    Cache: Name server caches all IP address for domain names that were requested

    recently. So that if requested again it responds immediately.

    Load Balancing: Large sites like www.msn.com can have multiple addresses for same

    domain name. Name servers currently return all IP addresses leaving PC to choose at random.

    But some name servers will now evaluate all addresses to find out he one with least load.

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    2.7 IPv6

    Due to recent concerns over the impending depletion of the current pool of Internetaddresses and the desire to provide additional functionality for modern devices, anupgrade of the current version of the Internet Protocol (IP), called IPv4, is in the process

    of standardization. This new version called IP Version 6 (IPv6), resolves unanticipatedIPv4 design issues and is poised to take the Internet into the 21 st Century.

    The current version of IP version 4 - is 20 years old. IPv4 has shown remarkable abilityto move to new technologies. But the ever-growing demand for IP addresses has madeIETF to propose a entirely new version to address some specific problems.

    2.7.1 Success of IP and Motivation

    The current version of IP has been extremely successful. IP has made it possible for theInternet to handle heterogeneous networks, dramatic changes in hardware technology,

    and extreme increases in scale. The demonstration of scalability is evident because thecurrent Internet includes millions of users around the world. The current version of IPhas also accommodated changes in hardware technology. Although the protocol wasdefined before local area network technologies became popular, the original design hascontinued to work well through several generations of hardware technologies. IP is nowused over networks that operate several orders of magnitude faster than the networksthat were in use when IP was designed. Furthermore, some modern networks offerframe sizes that are much larger than the frame sizes available when IP was defined.More important, IP works efficiently over such networks because it can take advantageof the increased frame size.

    Motivation for changeThe primary motivation for change arises from the limited address space. When IP wasdefined, only a few networks existed. The designers decided to use 32 bits for an IPaddress because doing so allowed the Internet to include over a million networks.However, the global Internet is growing exponentially, with the size doubling in less thana year. At the current growth rate, each of the possible network prefixes will soon beassigned, and no further growth will be possible. Thus, the primary motivation fordefining a new version of IP arose form the address space limitation - larger addressesare necessary to accommodate continued growth of the Internet.

    Secondary motivations for changes in IP have arisen from new Internet applications.For example, applications that deliver audio and video need to deliver data at regularintervals. To keep such information flowing through the internet without disruption, IPmust avoid changing routes frequently. Although the current IP datagram headerincludes a field that can be used to request a type of service, the protocol did not definea type of service that can be used for real time delivery of audio and video.

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    Support for audio and video - flow labels and quality of service allow audio andvideo applications to establish appropriate connections

    Extensible - new features can be added more easily

    2.7.4 IPv6 datagram format

    Figure 2.27: IPv6 datagram format

    As shown in the figure 2.27 above, an IPv6 datagram begins with a base header, whichis followed be zero or more extension headers, followed by data.

    Although it illustrates the general datagram structure, fields in the figure are not drawnto scale. In particular, some extension headers are larger than the base header, whileothers can be smaller. Furthermore, in many datagrams, the size of the data area ismuch larger than the size of the headers.

    IPv6 base header format

    Although it is twice large as an IPv4 header, the IPv6 base header contains lessinformation. As the figure in the next page shows, most of the space in the header isdevoted to two fields that identify the sender and the recipient. Each address occupies16 octets, four times more than an IPv4 address.

    There are 6 other fields in the base header. The VERS field identifies the protocol asversion 6. The PRIORITY field specifies the routing priority class. The PAYLOADLENGTH field specifies only the size of data being carried; the size of the header is

    excluded. The HOP LIMIT corresponds to the IPv4 TTL field. IPv6 interprets the HOPLIMIT strictly - the datagram is discarded if the HOP LIMIT counts down t zero beforethe datagram arrives at its destination.

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    Figure 2.28: IPv6 base header

    Field FLOW LABEL is intended for use with new applications that require performanceguarantees. The label can be used to associate a datagram with a particular underlyingnetwork path. The label is divided into 2 parts - one used to specify a traffic class, andthe other used to define a specific path. The traffic class specifies generalcharacteristics that the datagram needs. For example, to send interactive traffic onemight specify a general traffic class for low delay. To send real-time audio across an

    internet, however, a sender might request the underlying network hardware to establisha path that has delay less than 100 ms. When the path is established, the networksystem returns an identifier that the sender places in each datagram to be sent alongthe path. Routers use the FLOW LABEL field to route the datagram along theprearranged path.

    The NEXT HEADER field is used to specify the type of information that follows thecurrent header. For example, if the datagram includes an extension header, the NEXTHEADER field specifies the type of the extension header. If no extension header exists,the NEXT HEADER field specifies the type of data being carried in the datagram. Thefigure 2.29 shows this.

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    Figure 2.29: Use of NEXT header to specify type of

    informationParsing IPv6 headers

    If the value of the NEXT HEADER field corresponds to another header, IP softwareparses the header and interprets its contents. Once it finishes with the header, IP usesthe NEXT HEADER field to determine whether data or another header follows.

    Some header types have a fixed size. For example, a base header has fixed size ofexactly forty octets. To move to the item following a base header, IPv6 s/w simply adds40 to the address of the base header.

    Figure 2.30: The general form of an IPv6 options header

    Some extension headers do not have a fixed size. In such cases, the header mustcontain sufficient information to allow IPv6 to determine where the header ends. Forexample, figure above illustrates the general form of an IPv6 options header that carriesinformation similar to the options in an IPv4 datagram.

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    The options extension header illustrates one way IPv6 handles headers that do nothave a fixed size. When composing a datagram, the sender stores the length of theoptions header in field HEADER LEN. When the receiver encounters an optionsextension header, it uses this field to determine the location of the next item, and theNEXT HEADER field to determine the type.

    2.7.5 Fragmentation and Path MTU

    Although IPv6 fragmentation resembles IPv4 fragmentation, the details differ. Like IPv4,a prefix of the original datagram is copied into each fragment, and the payload length ismodified to be the length of the fragment. Unlike IPv4, however IPv6 does not includefields for fragmentation information in the base header. Instead, IPv6 places them in aseparate fragment extension header; the presence of the header identifies the datagramas a fragment.

    Figure 2.31: Fragmentation in IPv6

    As the figure above illustrates, each fragment is smaller than the original datagram. Aswith IPv4, the fragment size is chosen to be the MTU of the underlying network overwhich the fragments must be sent. Thus, the final fragment may be smaller than theothers because it represents the amount remaining after MTU-size pieces have beenextracted from the original datagram.

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    Unlike IPv4, in IPv6 a sending host is responsible for fragmentation. That is, hosts areexpected to choose a datagram size that will not require fragmentation; routers alongthe path that receive a large datagram will not fragment the datagram.

    The host must learn the MTU of each network along the path to the destination, and

    must choose a datagram size to fit the smallest. The minimum MTU along a path from asource to a destination is known as path MTU, and the process of learning the pathMTU is known as path MTU discovery.

    Path MTU discovery is an iterative procedure. A host sends a sequence of various sizedatagrams to the destination to see if they arrive without error. Once a datagram issmall enough to pass through without fragmentation, the host chooses a datagram sizeequal to the path MTU.

    2.7.6 Use of multiple headers

    There are 2 reasons for going for multiple headers: economy and extensibility.

    Partitioning the datagram functionality into separate headers is economical because itsaves space. Although IPv6 protocol includes many facilities, designers expect adatagram to use only a small subset. Because most datagrams only need a fewheaders, avoiding unnecessary header fields can save considerable space. Smallerdatagrams also take less time to transmit.

    Consider adding a new feature to a protocol. A protocol like IPv4 that used fixed headerformat requires a complete change - the header must be redesigned to accommodatefields needed to support the new feature. In IPv6, however, existing headers can remain

    unchanged. A new NEXT HEADER type is defined as well as a new header format.

    The chief advantage of placing a new functionality in a new header lies in the ability toexperiment with a new feature before changing all computers in the Internet. Forexample: suppose the owners of two computers wish to test new datagram encryptiontechnique. The two must agree on the details of an experimental encryption header.The sender adds the new header to a datagram, and the receiver interprets the headerin incoming datagrams. As long as the new header appears after the headers used forrouting, routers in the internet between the sender and receiver can pass the datagramwithout understanding the experimental header. Once a experimental feature provesuseful, it can be incorporated in the standard.

    2.7.7 IPv6 addressing and address notation

    IPv6 assigns a unique address for each connection between a computer and a physicalnetwork. Also like IPv4, IPv6 separates each such address into a prefix that identifiesthe network and a suffix that identifies a particular computer on the network.

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    Addresses do not have defined classes. Instead, the boundary between the prefix andsuffix can fall anywhere within the address and cannot be determined from the addressalone. Thus a prefix length must be associated with each address to enable s/w to knowwhere the prefix ends.

    IPv6 defines a set of special addresses that differ dramatically from IPv4 specialaddresses. IPv6 address is one of the three types:

    Unicast: The address corresponds to a single computer. A datagram sent to theaddress is routed along a shortest path to the computer.

    Multicast: The address corresponds to a set of computers, possibly at many locations;membership in the set can change at any time. When a datagram is sent to theaddress, IPv6 delivers one copy of the datagram to each member of the set.

    Anycast: The address corresponds to a set of computers that share a common address

    prefix. A datagram sent to the address is routed along a shortest path and thendelivered to exactly one of the computers.

    Anycast addressing was originally known as cluster addressing. The motivation for suchaddressing arises from a desire to allow replication of services. For example, acorporation that offers a service over the network assigns an anycast address to severalcomputers that all provide the service. When a user sends a datagram to the anycastaddress, IPv6 routes the datagram to one of the computers in the set. If a user fromanother location sends a datagram to the anycast address, IPv6 can choose to routethe datagram to a different member of the set, allowing both computers to processrequests at the same time.

    IPv6 address notation

    128-bit addresses unwieldy in dotted decimal; requires 16 numbers105.220.136.100.255.255.255.255.0.0.18.128.140.10.255.255

    Groups of 16-bit numbers in hex separated by colons

    -colon hexadecimal (or colon hex)

    Although an address that occupies 128 bits can accommodate Internet growth,

    writing such numbers can be unwieldy. For example consider a 128 bit numberwritten in dotted decimal notation shown above.

    To help reduce the number of characters used to write a address the designersof IPv6 propose using a more compact syntactic from known as colonhexadecimal notation in which each group of 16 bits is written in hexadecimalwith a colon separating groups. When the earlier example is written in colon hex,it becomes what is shown in the second bullet.

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    Zero-compression - series of zeroes indicated by two colons

    FF0C:0:0:0:0:0:0:B1

    FF0C::B1

    IPv6 address with 96 leading zeros isinterpreted to holdAs the above example illustrates, colon hex notation requires fewer characters toexpress an address. An additional optimization known as zero compressionfurther reduces the size. Zero compression replaces sequences of zeroes withtwo colons. For example consider the figure above.

    The large IPv6 address space and the proposed address allocation schememake zero compression especially important because the designers expectmany IPv6 addresses to contain strings of zeroes. In particular, to help ease of

    transition to the new protocol, the designers mapped existing IPv4 addresses intothe IPv6 address space. Any IPv6 address that begins with 96 zero bits containsan IPv4 address in the low order 32 bits.

    2.7.8 Differences between IPv4 and IPv6

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