Addressing in LANs Version 1.0 by Geoff Bennett This presentation discusses addressing concepts for LOCAL Area Networks. The primary difference between.

Addressing in LANsAddressing in LANsVersion 1.0Version 1.0

bybyGeoff BennettGeoff Bennett

This presentation discusses addressing concepts for LOCAL Area Networks. The primary difference between LAN and WAN addressing is that LANs are capable of supporting broadcast or multicast address destinations. WANs are not normally able to do this.

ADDR.PPTADDR.PPT

Networks allow us to transmit information between one computer and another.

Part of this procedure is the use of addressing to make sure messages get to the right place.

As we’ll see in this tutorial, addressing schemes exist at multiple layers of the OSI Model. A typical TCP/IP packet will contain addresses that are designed to be used at the MAC Layer, the Network Layer and the Transport Layer.

Physical LayerPhysical Layer

MAC LayerMAC Layer

LLC LayerLLC Layer

Network LayerNetwork Layer

Transport LayerTransport Layer

AddressingAddressingSchemesSchemes

The addresses are stored in specifically defined parts of the IP packet and the LAN frame.

The consistent position of addresses is a key factor that allows software to interpret addressing information correctly. In other words, if we put addresses in the wrong format, or in the wrong place, our communication software will not work correctly.

MAC Layer Addresses

Network Layer Addresses

Transport Layer Addresses

Why Do We Need Why Do We Need Addressing?Addressing?

In this diagram, let’s assume that Harry wants to send information to Sally.

Lets further assume that both computers are equipped with suitable interface circuits that allow them to insert messages into the network.

HarryHarry SallySally

If we just connect a cable between the two computers, then Harry’s software can simply push the information into the cable, and it will inevitably end up at the right place.

You may have even used such a configuration if you’ve ever downloaded information using modems, and software such as Kermit.


On a real network, such as Ethernet, Harry and Sally are not the only users of the communication channel.

All of the computers attached to this network share the same communication channel.

Addressing is used to ensure that messages between any two of these machines are not received by other users.


CurleyCurley LarryLarry MoeMoe

Physical Layer

MAC Layer

LLC Layer

Network Layer

Transport Layer

MAC Layer AddressingMAC Layer Addressing

MAC Layer Addressing is often explained in terms of security. In other words, all the stations are on the same network and MAC Addressing ensures that one stations cannot receive messages intended for another station. However, this is a naive way to think of MAC Addressing, since it is so easy to bypass this security.

Instead, we should regard MAC Addressing as a way to ensure that other LAN users are not forced to process messages that are actually being sent to someone else.


There are several phrases used to describe MAC Layer Addressing. They are all identical in meaning.

Local Wire Address is a slang phrase, and refers to the fact that the reason for the addressing scheme is to differentiate LAN stations that are attached to the same cable. I tend not to use this term because it is not such a good description in these days of LAN switches and multiport bridges.

•Local Wire AddressLocal Wire Address

Physical Address is the term used in RFC documents to describe the MAC Address. I believe it came into use because the MAC Address is tied to the physical host from which the frame originates, or to which it is directed.


•Physical AddressPhysical Address

MAC (Media Access Control) address is a term that’s used throughout the industry, and it’s the one I’ve grown used to. It is at the MAC Layer of the OSI Model that these addresses have significance.


•Physical AddressPhysical Address

•MAC AddressMAC Address

At the Physical Layer of the OSI Model, electrical signals are interpreted as a series of binary 1’s and 0’s.

The Physical Layer functions don’t make any attempt to interpret these bits in any way.

+5V

-5V

...1101011010...

At the MAC Layer, the 1’s and 0’s are interpreted into a structure called a Frame.

Frames are the lowest level collection of information on a LAN.

Frames can be quite long. On Ethernet, they are up to 1.5kB (about 12 000 bits), on Token Ring up to 18kB and on FDDI up to 4kB.

The smallest frame size is also specific to a given LAN technology. Ethernet has a minimum of 64 bytes.

Direction of TransmissionDirection of Transmission

FLAG FLAGDA SA CRCFRAMEFRAME

Frames have a structure that is specific to the LAN technology. Ethernet, Token Ring and FDDI frames are all slightly different in structure.

This diagram is a generic view of a frame.

The bits in the frame are transmitted in order from left to right. This is the typical convention used in most textbooks.


FLAGFLAG FLAGFLAGDADA SASA CRCCRC

The first feature of a frame is some form of delimiter, or flag. Flags are some special bit pattern, or line encoding, that allows the LAN circuits to identify the beginning and ending of the frame.



On Ethernet, the flag at the start of the frame is a series of 62 bits alternating 1 and 0, and then two bits set to 1. Ethernet and IEEE standards refer to this field as the preamble.

Another major use for the preamble is to allow LAN adapters to “lock on”, or synchronise with the clock signal that is contained within the bitstream encoding.

The ending delimiter is actually a “gap” in transmission - this must last at least 9.6 microseconds, but will be longer if no other station is ready to transmit. Ethernet and IEEE standards refer to this as the interframe gap.



In Token Ring and FDDI technologies, the flags are represented by special line coding.

For Token Ring, the coding is actually a controlled violation of the Manchester Encoding scheme.

For FDDI, special 5-bit symbol patterns are reserved for flags.



Towards the end of the frame is a field called the Cyclic RedundancyCheck (CRC). This is used to check for frame corruption.



At the beginning of the frame are the two MAC Address fields. The first of these fields is the Destination Address (DA), and the second is the Source Address (SA).

For Harry’s message to Sally, Harry would insert Sally’s MAC Address in the DA field, and his own MAC Address in the SA field.



MAC Address StructureMAC Address Structure

Although frames are specific to a given LAN technology, the most popular modern technologies (Ethernet, Token Ring and FDDI) all use the same address structure.


This MAC address structure is defined by the IEEE 802 committee, and is 48 bits long.

As you can see, representing these addresses in binary is a bit tedious, and so we normally write the address in hexadecimal. A 48 bit address can be written as 12 hex digits.

We use hex rather than decimal because there is a further structure to the 48 bit address.


0000000000000000000000000000000010110101...

48 bits48 bits

Here’s a typical IEEE MAC address, divided into hex digits. Each hex digit is the equivalent of 4 bits.

2 0 0 0 4 3 D 7 1 5 E4

2 0 0 0 4 3 D 7 1 5 E4

The first two bits of the address have a special significance.

The first bit is known as the Group/Individual (G/I) bit.

If this bit is clear (ie. 0), then the address is a Unicast address. This means that the frame is addressed to only one possible LAN interface.

If the G/I bit is set (ie. 1) then the frame is a Broadcast or Multicast.

Binary Representation= Binary Representation= 00010010

G/I BitG/I Bit

The second bit is known as the Global/Local (G/L) bit.

If this bit is clear, then this MAC address has been allocated from a block of addresses which is registered with the IEEE. In this case, no other LAN interface in the universe should have an identical address. In other words, this is a Globally Administered address.

If the bit is set, then this address was created by the local LAN administrator, and it may not be globally unique. In other words, it is a Locally Administered address.

2

Binary Representation= 0Binary Representation= 0001010

0 0 0 4 3 D 7 1 5 E4

G/L BitG/L Bit

2 0 0 0 4 3 D 7 1 5 E4

For globally administered addresses, the IEEE allocates a 24 bit address block to organisations that apply.

Once the block is allocated, the organisation is responsible for uniquely assigning addresses within its own block.

Large organisations (such as DEC and IBM) have multiple 24 bit blocks.

2 0 0 0 4 3 D 7 1 5 E4

Assigned by IEEEAssigned by IEEE

Allocated by OrganisationAllocated by Organisation

Here are a few examples of IEEE-assigned address blocks. A complete list can be found in the latest version of the “Assigned Numbers” RFC.

WellfleetWellfleet

ProteonProteon

SunSun

IBMIBM

DECDEC

CiscoCisco

OrganisationOrganisation

0000A2h0000A2h

000093h000093h

080020h080020h

08005A (et. al)08005A (et. al)

08002B (et. al.)08002B (et. al.)

00000Ch00000Ch

Address BlockAddress Block

MAC Addresses in ActionMAC Addresses in Action

Let’s say that Joe Bloggs Inc. apply to the IEEE and are given the 24 bit block “200043”. No other organisation will ever be given the same address block.

Joe Bloggs manufacture an Ethernet interface, and assign the remaining 24 bits. They then “install” this address into a permanent memory device (a PROM or PAL chip) on the interface.

No other LAN interface (even Token Ring or FDDI interfaces) should ever be assigned this address by Joe Blogss.

Address ChipAddress Chip

If we were always able to use Globally Administered addresses, we could be sure that no two machines in the world are using the same MAC address.

So you might think that MAC addresses are all we need to send LAN traffic between any two machines in the world.

Unfortunately this is not true, for two reasons...Local Addressing and Scaleability.

Local Addressing - How?Local Addressing - How?

If IEEE-registered addresses are installed in every LAN card, how can we use local addressing?

The answer is simple. When the chipset on a LAN interface is activated, it reads the MAC address from the chip. Communication software can then write a different address to the chip, and so allow locally administered addressing.

Some chipsets are even able to accept multiple MAC addresses to operate simultaneously.

Address ChipAddress Chip

Local Addressing - Why?

For a locally administered address, we set the G/L bit to “warn” other end stations that this address does not have global significance.

But why should we bother to use local addresses when the IEEE procedure guarantees that addresses will never be duplicated.

There is no single answer to this question, just a set of industry stories...

6

Binary Representation= 0Binary Representation= 0111010

0 0 0 4 3 D 7 1 5 E4

G/L BitG/L Bit

If we adopt local address administration, we may be able to build networks within our own domains of control. Perhaps this domain consists of the building in which we work, or even just the floor where our workgroup is located.

In order to connect local address domains without worrying about MAC Address duplication, we use Routers.

Routers make their decisions based on Network Layer Addressing.

LocalAddressDomain

RouterRouter

RouterRouterLocal

AddressDomain

Local Addressing - ScaleabilityLocal Addressing - Scaleability

Physical Layer

MAC Layer

LLC Layer

Network Layer

Transport Layer

Network Layer AddressingNetwork Layer Addressing

Network Layer addresses are found inside the Data Field portion of the frame.

MAC Layer AddressesMAC Layer Addresses

Network Layer AddressesNetwork Layer Addresses

Transport Layer AddressesTransport Layer Addresses

IP Addressing Basics• IPv4 addresses are usually written as four separate

numbers delineated by a period – For example: 101.209.33.17

• This way of representing an IP address is called the dotted-quad notation

• Each number in the four-number group is represented as an 8-bit octet in an IPv4 header– For example: 101.209.33.17 would be represented as:

– 01100101 11010001 00100001 00010001

More IP Addressing Basics

• In IPv4, each 32-bit IP address is subdivided into network and host/node portions

• The composition of the first four bits in the IP address specifies whether the network portion is 1, 2, or 3 bytes in length– These four bits determine whether the host/node

has a Class A, B, C, D, E address (see Table 4-1)

For IP, the structure of the address is relatively simple.

We take a 32 bit address.


We take a 32 bit address

Divide it into 4, 8-bit fields.

the dotted-quad notation



Divide it into 4, 8-bit fields..

Then we evaluate each field separately in decimal.

202202 3434 1919 88



Divide it into 4, 8-bit fields.

Then we evaluate each field separately in decimal.

And we write down these values with the individual byte-fields separated by dots. This is called dotted decimal notation.

202202 3434 1919 88

IP Addresses

0network host

10 network host

110 network host

1110 multicast address

A

B

C

D

class1.0.0.0 to127.255.255.255

128.0.0.0 to191.255.255.255

192.0.0.0 to223.255.255.255

224.0.0.0 to239.255.255.255

32 bits

given notion of “network”, let’s re-examine IP addresses:

“class-full” addressing:

IPv4 Address Classes

IPv4 Classes

Default subnet masks.

The logical AND operation applied to 2 bits and the results.

Default subnet mask applied to a Class C address.

A subnet mask applied to a Class C address.

IP Addressing: introduction IP address: 32-bit

identifier for host, router interface

interface: connection between host/router and physical link routers typically have

multiple interfaces hosts may have

multiple interfaces IP addresses

associated with each interface

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

223.1.1.1 = 11011111 00000001 00000001 00000001

223 1 11

IP Addressing IP address:

network part (high order bits)

host part (low order bits)

What’s a network ? (from IP address perspective) device interfaces with

same network part of IP address

can physically reach each other without intervening router

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

network consisting of 3 IP networks(for IP addresses starting with 223, first 24 bits are network address)

LAN

IP Addressing How to find the

networks? Detach each

interface from router, host

create “islands of isolated networks

223.1.1.1

223.1.1.3

223.1.1.4

223.1.2.2223.1.2.1

223.1.2.6

223.1.3.2223.1.3.1

223.1.3.27

223.1.1.2

223.1.7.0

223.1.7.1223.1.8.0223.1.8.1

223.1.9.1

223.1.9.2

Interconnected system consisting

of six networks

IP addresses: how to get one?

Q: How does host get IP address?

hard-coded by system admin in a file Wintel: control-panel->network->configuration->tcp/ip-

>properties UNIX: /etc/rc.config

DHCP: Dynamic Host Configuration Protocol: dynamically get address from as server “plug-and-play”

IP addresses: how to get one?

Q: How does network get network part of IP addr?

A: gets allocated portion of its provider ISP’s address space

ISP's block 11001000 00010111 00010000 00000000 200.23.16.0/20

Organization 0 11001000 00010111 00010000 00000000 200.23.16.0/23 Organization 1 11001000 00010111 00010010 00000000 200.23.18.0/23 Organization 2 11001000 00010111 00010100 00000000 200.23.20.0/23 ... ….. …. ….

Organization 7 11001000 00010111 00011110 00000000 200.23.30.0/23

IP addressing: the last word...

Q: How does an ISP get block of addresses?

A: ICANN: Internet Corporation for Assigned Names and Numbers

allocates addresses manages DNS assigns domain names, resolves disputes

Reserved IP Addresses

• The developers of the IPv4 addressing scheme reserved three blocks of addresses for networks that would not be connected to the Internet– These are identified and defined in RFC 1918

• Reserved address ranges are illustrated in Table 4-3

Table 4-3

Domain Names and URLs

• When a domain name is included in a URL, it must be resolved to an IP address

• This is done by the Internet’s Domain Name System (DNS)

• Domain names and their IP addresses are stored in databases on domain name servers

• When a domain name must be resolved, a message is sent to the closest domain name server to obtain the IP address. If that server does not know the IP address, it sends a request to other domain servers for the information

• Once the IP address for a domain name is known, the host/node inserts the IP address as the destination address for the packet so that it can be routed to appropriate recipient

Subnet Addressing

• Because there is a limited number of available IPv4 addresses, IPv4 developers provided mechanisms for sharing a single network address among two or more subnets– These mechanisms are described in RFC 950

– RFC 950 enables class A, B, and C networks to be split into smaller networks that use the same network assignment numbers

Subnetting Advantages

• Subnetting has the following advantages:– It simplifies network administration; each network

segment can be maintained independently and efficiently

– Intranets can be restructured without affecting the overall network’s interfaces with the Internet and other external networks

– Because intranet subnetting is not visible to external networks it can be used to enhance the overall security of the organization’s networks

Subnetting Basics

• Subnetting enables network managers to extend the network portion of IPv4 addresses by taking away a portion of the host/node portion of the IP address

• The portion that is taken away is used as a subnet identifier

• This is illustrated in Figure 4-4

Figure 4-4

Subnet Masks• A subnet mask is a binary bit pattern that is stored in hosts, nodes, and

routers• It is matched up with an incoming packet’s destination IP address to

determine whether to accept or reject the packet• Every TCP/IP network host/node or router stores a subnet mask along with

its IP address (see Figure 4-6)• The subnet mask specifies which bits in an IP address should be treated as

an extended network address (network + subnet) and which bits represent the host/node portion of the address

• Default subnet masks exists for class A, B, and C networks (see Table 4-9)• Table 4-10 summarizes alternative class C subnet masks• Figure 4-5 illustrates how a subnet mask is used to decompose an IPv4

address into its subnet and host/node addresses

Figure 4-6

Table 4-9

Table 4-10

Figure 4-5

Static vs. Dynamic IP Addresses

• Host/node addresses can be allocated in one of two ways:– Static assignments– Dynamic assignments

• Static IP addresses are permanently assigned to hosts and node– Servers and routers are typically assigned static IP addresses– These can be assigned to hosts/nodes through manual configuration or

by always assigning the same IP address to a particular host/node when it comes online

• Dynamic IP addresses are automatically assigned to client stations in a TCP/IP network when they come online– DHCP servers assign dynamic IP addresses to clients

Internet Addressing in LANs

• Additional addressing processes take place when the host/node that needs to connect to the Internet is in a LAN

• In LANs, physical (MAC) addresses (the address of the computers’ network interface cards) are used for message delivery

• When a LAN host/node has both an IP address and a MAC address, an incoming IP packet can only be delivered to the computer after the IP address has been translated to a MAC address

• The protocol that performs this function is address resolution protocol (ARP)

Address Resolution Protocol (ARP)

• ARP servers maintain tables that contain host/node IP addresses and corresponding MAC addresses (see Table 4-12)

• If the destination node’s IP address is in the ARP table, it extracts the corresponding MAC address and uses it to build the MAC header needed to send the message to the node

• ARP is found at the Internet layer of the TCP/IP protocol stack (see Figure 4-10) but is often described as overlapping the Internet and media access layers because of its role in translating IP to MAC addresses

Table 4-12

Figure 4-10

RARP Illustrated (Example I)

Ethernet : 0800.0020.1111IP = ????

IP = 131.108.3.1

I know who you are, here’s your IP

address

Here’s my MACaddress. What is my

IP address ?

131.108.3.20800.0020.1111

RARP Illustrated (Example II)

Ports and Sockets

• Once received by the destination host/node, a packet progresses up the layers of the TCP/IP protocol stack and is directed to the appropriate application

• Port numbers are included in TCP or UDP headers to identify the application layer protocol that generated the data in the packet

• Some port numbers are permanently assigned to applications/services (see Table 4-15)

• The combination of an IP address and a port number is called a socket

– For example, the socket notation for a Web page request on a Web server whose IP address is 141.165.231.193 would be 141.165.231.193:80

Examples of Well-Known PortsTable 4-15

IP addresses have an additional structural element. Part of the address is reserved to indicate the Network ID, while the semainder of the address represents the Host ID. The relative sizes of the Network and Host ID fields vary with the class of IP address.

Using the Network ID, routers can direct traffic over multiple hops until it reaches the correct network.

The final router in the path will use the Host ID to perform an Address Resolution, and find out the correct MAC address of the destination host.

202202 3434 1919 88

Network IDNetwork ID Host IDHost ID

In the case of the worldwide Internet, there are over one million hosts already attached, and the connection rate is still increasing.

Without a hierarchical form of addressing, then internetwork routers would need to remember where every individual host was located.

With hierarchical addressing, each router only needs to track the hosts that are connected to networks on the router.

Hierarchical addressing is used in another global network - the Telephone System.

HH

H

H H

HH

H

This is a telephone number in the USA. It has 7 digits, which means that up to 10 million subscribers can be addressed individually.

This is a lot, but not enough for a national, or international addressing scheme.

Even the seven digits are actually divided into the Local Exchange, and the Subscriber Extension.

663663 6676 6676

International Prefix

Area Code

Local ExchangeLocal Exchange

Subcriber ExtensionSubcriber Extension

To extend the numbering scheme, US numbers add an area code.

Area codes are three digits long.

508508 663663 6676 6676

International Prefix

Area CodeArea Code



If we want to call this number from outside the US, we need to add the International Prefix. For the USA, this is 1, for the UK, 44, for Germany 49 etc.

11 508508 663 663 66766676

International PrefixInternational Prefix

Area CodeArea Code



This hierarchical numbering scheme is essential to the telephone system. By isolating the scope of individual telephone numbers, we gain a number of advantages.

First, human users of the system only need remember seven digits for any local number.

Second, a national PTT can adopt any reasonable internal structure for its numbering. It is “protected” from address duplication and confusion by the International Prefix.

Finally, and most important, any given telephone exchange only needs to know about addresses below it the hierarchy.

Local ExchangeLocal Exchange(or PBX)(or PBX)



NationalNationalExchangeExchange

InternationalInternationalExchangeExchange

NationalNationalExchangeExchange

Ext.Ext.

Ext.

I’d like to concentrate on this final advantage, because it’s the primary reason we use hierarchical addressing in data networks.

The most obvious evidence of address hierarchy is that used in the Internet for protecting one subscriber from routing errors made by another subscriber. This concept is, of course, the Autonomous System (AS).

Internet-attached routers must recognise AS concepts, and must terminate local routing protocol updates such as RIP or OSPF.

BackboneBackboneRouterRouter

Joe Bloggs’Joe Bloggs’AutonomousAutonomous

SystemSystem


Jane Doe’sJane Doe’sAutonomousAutonomous

SystemSystem

InternetInternetRouterRouter


TheTheInternetInternet

BackboneBackboneRouterRouterBackboneBackbone

RouterRouterBackboneBackboneRouterRouter

BackboneBackboneRouterRouterBackboneBackbone

RouterRouterBackboneBackboneRouterRouter

The next level in the hierarchy are the Backbone Routers, used to build the Corporate Backbone.

Backbone routers operate within an AS, but may need to maintain large routing tables depending on the size of the individual organisation.


TheTheInternetInternet





Corporate BackboneCorporate Backbone

At the lowest level of router hierarchy are the Access Routers. These devices are much smaller, less powerful, and cheaper than their more complex cousins.

Access Routers may be used to connect a single LAN workgroup into the Corporate Backbone, and don’t need to maintain complex routing tables.

Regardless of the type of router used, all of these devices make their switching decisions based on the Network Layer addressing I have just described.

AccessAccessRouterRouter



Finally I’d like to look at Transport Layer addressing.

Just to recap, we can say that MAC Layer addressing allows us to transfer messages between two hosts on the same cable.

Network Layer addressing extends this communication ability so that we can cross multiple intermediate networks to get from one host to another. Network Layer addressing is also scaleable because the network designer can choose the addresses in a hierarchical way.

Physical Layer

MAC Layer

LLC Layer

Network Layer

Transport LayerTransport Layer AddressingTransport Layer Addressing

Here we see a LAN frame heading towards a PC from the network. MAC and Network Layer addressing have got the frame this far, but now there’s a problem.

There are two possible communication programs running in the PC - Program 1 and Program 2.

The MAC and IP addresses on the PC only identify the machine itself, not the program to which the packet should be sent.

From theFrom theNetworkNetwork

Program 1Program 1

Program 2Program 2

To differentiate between these programs, we use Transport Layer addressing.

Note that it’s not really practical to use IP addresses on a per-program basis for a couple of good reasons. First of all, you’d have to register each program with an IP address when it started. Because IP addresses are assigned manually, the Network Administrator would have to limit the number of programs you can run from a machine so she would be able to pre-assign your IP addresses.

More critically, there are too few IP addresses to really do this in practice.

From theFrom theNetworkNetwork

Program 1Program 1

Program 2Program 2

To Summarize...To Summarize...

MAC Layer Addresses are used to allow private communication between specific hosts, even though they share the same communication channel with many other systems.


MAC LayerMAC Layer

Network Layer Addressing allows communication between hosts regardless of the type of network (or networks) that are used to connect the hosts.


MAC LayerMAC Layer


Transport Layer Addressing allows a specific application process running in a host computer to communicate with an equivalent process running in another host.


MAC LayerMAC Layer


Transport LayerTransport Layer

The EndThe End

Addressing in LANs Version 1.0 by Geoff Bennett This presentation discusses addressing concepts for LOCAL Area Networks. The primary difference between.

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