TCP/IP Internetworking Chapter 8. 8-2 Recap Single Networks (Subnets) –Chapters 4 and 5 covered single LANs –Chapters 6 and 7 covered residential Internet

TCP/IP Internetworking

Chapter 8

8-2

Recap

• Single Networks (Subnets)

– Chapters 4 and 5 covered single LANs

– Chapters 6 and 7 covered residential Internet access and single WANs

• Internets– Connect multiple single networks using routers

– 70%-80% of internet traffic follows TCP/IP standards

– These standards are created by the IETF

– Chapter 10 looks in more detail at TCP/IP management

8-3

Figure 2-8: Hybrid TCP/IP-OSI Architecture

General Purpose Layer Specific Purpose

Application-application communication

Application (5) Application-application interworking

Transmission across an internet

Transport (4) Host-host communication

Internet (3) Packet delivery across an internet

Transmission across a single network (LAN or WAN)

Data Link (2) Frame delivery across a network

Physical (1) Device-device connection

Recap

TCP/IP standards dominate at theinternet and transport layers—transmission across an internet

8-4

Figure 2-11: Internet and Transport Layer, Cont.

Transport Layerend-to-end (host-to-host)

TCP is connection-oriented, reliableUDP is connectionless and unreliable

Internet Layer(usually IP)

hop-by-hop (host-router or router-router)connectionless, unreliable

Router 1 Router 2 Router 3

Client PCServer

Recap

8-5

Frames and Packets

• Messages at the data link layer are called frames

• Messages at the internet layer are called packets

• Within a single network, packets are encapsulated in the data fields of frames

FrameHeader

Packet(Data Field)

FrameTrailer

Recap

8-6

Frames and Packets

• In an internet with hosts separated by N networks, there will be:

– 2 hosts

– One packet (going all the way between hosts)

• One route (between the two hosts)

– N frames (one in each network)

Recap

8-7

Figure 2-21: Combining Horizontal and Vertical Communication

Int

App

DL

Trans

Phy

Int

Trans

IntInt

SourceHost

DestinationHost

Switch1

Switch2

Router1

Switch3

Router2

Transmission Control Protocol (TCP)Or User Datagram Protocol (UDP)

Internet Protocol(IP)

Recap

IP

8-8

Figure 8-1: Major TCP/IP Standards

5 ApplicationUser Applications

HTTP SMTPMany

OthersDNS

RoutingProtocols

ManyOthers

Supervisory Applications

TCP UDP4 Transport

IP3 InternetMPLS

ARP

None: Use OSI Standards2 Data Link

None: Use OSI Standards1 PhysicalInternetworking is done at the internet and transport layers.

There are only a few standards at these layers.We will look at the shaded protocols in this chapter.

ICMP

8-9

Figure 8-1: Major TCP/IP Standards, Continued

5 ApplicationUser Applications

HTTP SMTPMany

OthersDNS

RoutingProtocols

ManyOthers

Supervisory Applications

TCP UDP4 Transport

IP3 Internet ICMP ARP

None: Use OSI Standards2 Data Link

None: Use OSI Standards1 Physical At the application layer, there areuser applications and supervisory applications.

We will look at two TCP/IP application layer supervisory applications in this chapter.

IP Addresses

32-Bit Strings

Dotted Decimal Notation for Human Reading(e.g., 128.171.17.13)

8-11

Figure 8-3: Hierarchical IP Address

128.171.17.13

Network Part (not always 16 bits)

Subnet Part (not always 8 bits)

Host Part (not always 8 bits)

Total always is 32 bits

UH Network (128.171)

CBA Subnet (17)Host 13

The Internet

Figure 8-3: Hierarchical IP Address

IP addresses are notsimple 32-bit numbers.

They usually have 3 parts.

Consider the example128.171.17.13

8-12

Hierarchical Addressing

• Hierarchical Addressing Brings Simplicity

– Phone System

• Country code-area code-exchange-subscriber number

• 01-808-555-9889

– Long-distance switches near the top of the hierarchy only have to deal with country codes and area codes to set up circuits

– Similarly, core Internet routers only have to consider network or network and subnet parts of packets

Router Operation

8-14

Figure 8-4: Border Router, Intrernal Router, Networks, and SubnetsFigure 8-4: Border Router, Internal Router, Networks, and Subnets

ISP Network60.x.x.x

Subnet 192.168.2.x

Subnet 192.168.3.x

Subnet192.168.1.xInternal

Router

BorderRouter

CorporateNetwork

192.168.x.x

Border routers connect different Internet networks(In this case, 192.168.x.x and 60.x.x.x).

An “x” indicates anything.

8-15

Figure 8-4: Border Router, Internal Router, Networks, and SubnetsFigure 8-4: Border Router, Internal Router, Networks, and Subnets

ISP Network60.x.x.x

Subnet 192.168.2.x

Subnet 192.168.3.x

Subnet192.168.1.xInternal

Router

BorderRouter

CorporateNetwork

192.168.x.x

Internal routers connect different subnets in a network.In this case, the three subnets are boxed in red:

192.168.1.x, 192.168.2.x, and 192.168.3.x.

8-16

Figure 8-5: Multiprotocol Routing

MultiprotocolRouter

X TCP/IP

TCP/IP

IPX/SPX

SNA

WWWServer

EdgeRouter

Z

Site ASite B

Mainframe

InternalRouter

Y

EthernetLAN 1

EthernetLAN 2

EthernetLan 3

The Internet

OldNetWareServer

UNIXServer

Figure 8-5: Multiprotocol Routing

Real routers must handle multipleinternet and transport layer architectures—

TCP/IP, IPX/SPX, SNA, etc.We will only look at TCP/IP routing

8-17

Figure 8-6: Ethernet Switching Versus IP Routing

A1-44-D5-1F-AA-4CSwitch 1, Port 2 B2-CD-13-5B-E4-65

Switch 1, Port 7

Port 7 on Switch 2to Port 4 on Switch 3

Port 5 on Switch 1to Port 3 on Switch 2

Switch2

Switch1

Switching Table Switch 1

Port Station2 A1-44-D5-1F-AA-4C7 B2-CD-13-5B-E4-655 C3-2D-55-3B-A9-4F5 D4-47-55-C4-B6-9F5 E5-BB-47-21-D3-56

Ethernet Switching

Destination address is E5-BB-47-21-D3-56.Ethernet switches are arranged in a hierarchy.

So there is only one possible path between hosts.So only one row can match an Ethernet address.

Finding this row is very simple and fast.So Ethernet switching is inexpensive per frame handled.

One Correct Row

8-18

Figure 8-6: Ethernet Switching Versus IP Routing

Network60.x.x.x

Packet to 60.3.47.129

Router B

Router C

Interface1

Interface2

Network60.x.x.x

IP Routing

Network60.3.x.x

Route

123456

IP AddressRange

60.3.x.x128.171.x.x60.3.47.x10.5.3.x

128.171.17.x10.4.3.x

Metric

928622

Router A

Routing Table for Router A

Host60.3.45.129

Next-HopRouter

BBCB

LocalC

Routing

Matches

Host60.3.47.x

Because of multiple alternative routes in router meshes,routers may have several rows that match an IP address.

Routers must find All matches and then select the BEST ONE.This is slow and therefore expensive compared to switching.

8-19

Figure 8-7: The Routing Process

• Routing

– Processing an individual packet and passing it on its way is called routing

• Router ports are called interfaces

• Packet arrives in one interface

• The router sends the packetout another interface

8-20


• The Routing Table

– Each router has a routing table that it uses to make routing decisions

– Routing Table Rows

• Each row represents a route for a RANGE of IP addresses—often a network or subnet

• All packets with addresses in this range are routed according to that row

RouteIP Address Range

Governed by the routeMetric

Next-HopRouter

1 60.3.x.x 9 B

8-21


• The Routing Table

– Routing Table Columns

• Row (route) number: Not in real routing tables

• IP address range governed by the row

• Metric for the quality of the route

• Next-hop router that should get the packet next if the row is selected as the best match

Route IP AddressRange

Metric Next-HopRouter

1 60.3.x.x 9 B

2 128.171.x.x 2 B

8-22


• A Routing Decision

– The router looks at the destination IP address in an arriving packet (in this case, 60.3.47.12).

– 1. The router determines which rows match (have an IP address range containing the packet’s destination IP address)

• The router must check ALL rows for possible matches



1 60.3.x.x 9 B

2 128.171.x.x 2 B

Arriving Packet60.3.47.12

Match

No Match

8-23



– 2. After finding all matches, the router then determines the BEST-MATCH row

• 2A. Selects the row with the LONGEST MATCH– 60.3.x.x has 16 bits of match– 60.3.47.x has 24 bits of match so is a better match

• 2B. If two or more rows tie for the longest match, router uses the METRIC column value

– If cost, lowest metric value is best– If speed, highest metric value is best– Etc.

8-24



– 3. After selecting the best-match row, the router sends the packet on to the next-hop router indicated in the best-match row—Next-Hop Router B in this example.



1 60.3.x.x 9 B

2 128.171.x.x 2 B

Best-Match Row

Send Packetout to

NHR B

A More Detailed Look at Routing Decisions

Box

8-26

Figure 8-8: Detailed Row-Matching Algorithm

• Routing Table

IP Address Range

Row Destination Mask … … …

1 10.7.3.0 255.255.255.0 … … …

2 … … … … …

3 … … … … …

Box

Actually, the table does not really have an “IP Address Range” column.Instead, it has two columns to indicate the IP address range:

Destination (an IP address) and a mask

8-27


• 1. Basic Rule of Masking

– Information Bit 1 0 1 0

– Mask Bit 1 1 0 0

– Result 1 0 0 0

• Where mask bits are one, the result gives the original IP address bits

• Where mask bits are zero, the result contains zeros

Box

8-28


• 2. Example

– Address (partial) 10101010 11001110

– Mask 11111000 00000000

– Result 10101000 00000000

Box

8-29


• 3. Common 8-bit Segment Values in Dotted Decimal Notation– Segment Decimal Value

00000000 0

11111111 255

• 4. Example– 255.255.255.0 is 24 ones followed by 8 zero

– 255.255.255.0 is also called /24 in “prefix notation”

Box

8-30


• Example 1: A Destination IP Address that is in the Range

• Destination IP Address of Arriving Packet 10.7.3.47

• Apply the Mask 255.255.255.0

• Result of Masking 10.7.3.0

• Destination Value 10.7.3.0

• Does Destination Value Match the Masking Result? Yes

• Conclusion Row 1 is a

match.


1 10.7.3.0 255.255.255.0 … … …

Box

8-31


• Example 2: A Destination IP Address that is NOT in the Range

• Destination IP Address of Arriving Packet 10.7.5.47

• Apply the Mask 255.255.255.0

• Result of Masking 10.7.5.0

• Destination Value 10.7.3.0

• Does Destination Value Match the Masking Result? No

• Conclusion Row 1 is NOT a

match.


1 10.7.3.0 255.255.255.0 … … …

Box

8-32

Figure 8-9: Interface and Next-Hop Router

• Switches

– A switch port connects directly to a single computer or another switch

– Sending the frame out a port automatically gets it to the correct destination

Frame

Box

8-33


• Routers

– Router ports (interfaces) connect to subnets, which have multiple hosts and that may have multiple routers

– The packet must be forwarded to a specific host or router on that subnet

Subneton RouterInterface

IPPacket

Next-HopRouter

Host

Host

Box

Next-HopRouter

8-34


RouterForwardingPacket


IP Subnet onInterface (Port) 5

PossibleNext-HopRouter

PossibleDestinationHost

Packet must be sent toa particular host orrouter

Router A Router B

Packet to Router B out Interface 5

PossibleNext-HopRouter

Router C

Box

Best-match row has both an interface (indicating a subnet)and also a next-hop router value to indicate a host or router on the subnet.

(Not just a Next Hop Router Column)

Interface (port) Next-Hop Router

Next-Hop Router

Dynamic Routing Protocols

Routing Table Information

Dynamic Routing Protocol

8-36

Figure 8-10: Dynamic Routing Protocols

• Routing

– How do routers get their routing table information?

– Routers constantly exchange routing table information with one another using dynamic routing protocols

– Note that the term routing is used in two ways In TCP/IP

• For IP packet forwarding and

• For the exchange of routing table information through routing protocols

Routing Table Information

Dynamic Routing Protocol

8-37


• Autonomous System– An organization’s internal network (internet)

• Exterior Dynamic Routing Protocols

– Between Autonomous Systems, companies use an exterior dynamic routing protocol

– The dominant exterior dynamic routing protocol is the Border Gateway Protocol (BGP)

• Gateway is an obsolete name for router

– Company is not free to choose whatever exterior routing protocol it wishes

8-38


• Interior Dynamic Routing Protocols

– Within an Autonomous System, firms use interior dynamic routing protocols

– Can select their own interior dynamic routing protocol

– Routing Information Protocol (RIP) for small internets

– Open Shortest Path First (OSPF) for larger internets

– Enhanced Interior Gateway Routing Protocol (EIGRP)• Non-TCP/IP proprietary CISCO protocol• Can handle multiple protocols, not just TCP/IP

8-39


Autonomous System

InternalRouter

BGP Is an Exterior DynamicRouting ProtocolAutonomous System

RIP,OSPF, orEIGRP

RIP,OSPF, orEIGRP

InternalRouter

BorderRouter

BorderRouter

RIP, OSPF, and EIGRPInterior Dynamic Routing Protocols


Recap

The Address Resolution Protocol (ARP)

8-41

Figure 8-12: Address Resolution Protocol (ARP)

OriginatingRouter

Host110.19.8.47

does not respond toARP Request.

1.Broadcast ARP Request Message:

"IP Host 110.19.8.17,what is your 48-bit MAC address?"

Host110.19.8.17

replies.2.

ARP Response Message:"My MAC address is A7-23-DA-95-7C-99".


Router B110.19.8.

does not reply

ARP Cache:Known

IP address-EthernetAddress

Pairs

The Situation:The router wishes to pass the packet to the

destination host or to a next-hop router.The router knows the destination IP address of the target.

The router must learn the target’s MAC layer addressin order to be able to send the packet to the target in a frame.

The router uses the Address Resolution Protocol (ARP)

Packet

Frame

8-42

OriginatingRouter

Host110.19.8.47




Host110.19.8.17

replies.2.



Router B110.19.8.

does not reply

ARP Cache:Known


Pairs


1: Router broadcasts ARP Request to all hosts and routers on the subnet.

8-43

OriginatingRouter

Host110.19.8.47




Host110.19.8.17

replies.2.



Router B110.19.8.

does not reply

ARP Cache:Known


Pairs


This is theDestination host

2: ARP Reply sent by the host with the target IP address.

Other hosts ignore it.

8-44

OriginatingRouter

Host110.19.8.47




Host110.19.8.17

replies.2.



Router B110.19.8.

does not reply

ARP Cache:Known


Pairs


3.Router puts the MAC address in its ARP cache;

uses it for subsequent packets to the host

Multiprotocol Label Switching (MPLS)

8-46

Figure 8-13: Multiprotocol Label Switching (MPLS)

• Routers are Connected in a Mesh

– Multiple alternative routes make the routing decision for each packet very expensive

• PSDNs (Chapter 7) also are Arranged in a Mesh

– However, a best path (virtual circuit) is set up before transmission begins

– Once a VC is in place, subsequent frames are handled quickly and inexpensively

• MPLS Does Something Like this for Routers

8-47


• MPLS Adds a Label Before Each Packet

– Label sits between the frame header and the IP header

– Contains an MPLS label number

– Like a virtual circuit number in a PSDN frame

– Label-switching router merely looks up the MPLS label number in its MPLS table and sends the packet back out

Data LinkHeader

MPLSLabel

IPPacket

8-48


• Advantages of MPLS

– Router does a simple table lookup. This is fast and therefore inexpensive per packet handled

• As fast as Ethernet switching!

– Can use multiple label numbers to give traffic between two sites multiple levels of priority or quality of service guarantees

– MPLS supports traffic engineering: balancing traffic on an internet

Label Port

1 3

8 2

8-49

Figure 8-13: Multiprotocol Label Switching (MPLS)Figure 8-13: Multiprotocol Label Switching (MPLS)

Label-SwitchedPath

Label-SwitchingRouter 1




Label-SwitchingRouter 5Packet Label

Legend

Label-Switching TableLabelACF

Interface113

MPLS reduces forwarding costs and permits traffic engineering,including quality of service and traffic load balancing

First routeradds the label

Last routerdrops the label

The Domain Name System (DNS)

8-51

Figure 8-14: Domain Name System (DNS) Hierarchy

(root)

cnn.commicrosoft.comhawaii.edu

.com .uk.ie.edu .net

Top-LevelDomainNames

Second-LevelDomainNames

Subnet Namecba.hawaii.edu

voyager.cba.hawaii.edu ntl.cba.hawaii.eduHost Names


.nl.org .auA domain is a group of resources

under the control of an organization.

The domain name system is ageneral system for managing names.

It is a hierarchical naming system.

Queries to a DNS server can getInformation about a domain.

8-52


(root)








.nl.org .au

The highest level (0) is called the root.There are 13 DNS Root Servers.They point to lower-level servers.

8-53


(root)








.nl.org .au

Top-level domains aregeneric TLDs (.com, .net., .org, etc.) or

country TLDs (.ca, .uk, .ie, etc.)

8-54


(root)








.nl.org .au

Organizations seekgood second-level domain

names

cnn.commicrosoft.com

hawaii.eduetc.

Firms get them fromaddress registrars

8-55


(root)








.nl.org .au

Host names are the bottomof the DNS hierarchy.

A DNS request for a host namewill return its IP address.

The Internet Control Message Protocol (ICMP)

8-57

Figure 8-15: Internet Control Message Protocol (ICMP) for Supervisory Messages

RouterHost UnreachableError Message

Echo Request(Ping)

EchoResponse


IPHeader

ICMPMessage

ICMP messages are encapsulated in thedata fields of IP packets.

There are no transport orApplication layer headers or messages

ICMP is the supervisory protocolat the internet layer.

8-58



Echo Request(Ping)

EchoResponse


IPHeader

ICMPMessageWhen an error occurs, the device

noting the error may try to respond with anICMP error message describing the problem.

ICMP error messages often are not sentfor security reasons because

attackers can use them to learn about a network

8-59



Echo Request(Ping)

EchoResponse


IPHeader

ICMPMessage

To see if another host is active, a hostcan send the target host an ICMP echo

message (called a ping).

If the host is active, it will send back anecho response message confirming that it is active.

Dynamic Host Configuration Protocol (DHCP)

From Chapter 1

8-61

Figure 8-16: Dynamic Host Configuration Protocol (DHCP)

• DHCP Gives Each Client PC at Boot-Up:

– A temporary IP Address (we saw this in Chapter 1)

– A subnet mask

– The IP addresses of local DNS servers

• Better Than Manual Configuration

– If subnet mask or DNS IP addresses change, only the DHCP server has to be updated manually

– Client PCs are automatically updated when they next boot up

The Internet Protocol (IP)

Versions 4 and 6

8-63

Figure 8-17: IPv4 and IPv6 Packets

IP Version 4 Packet

Version(4 bits)Valueis 4

(0100)

HeaderLength(4 bits)

Flags(3 bits)

Time to Live(8 bits)

Header Checksum(16 bits)

Diff-Serv(8 bits)

Total Length(16 bits)

Length in octets

Bit 0 Bit 31

Identification (16 bits)Unique value in each original

IP packet

Fragment Offset (13 bits)Octets from start of

original IP fragment’sdata field

Protocol (8 bits)1=ICMP, 6=TCP,

17=UDP

IPv4 is the dominant version of IP today.The version number in its header is 4 (0100).

The header length and total length field tell the size of the packet.

The Diff-Serv field can be used for quality of service labeling.(But MPLS is being used instead by most carriers)

8-64


IP Version 4 Packet


(0100)


Flags(3 bits)



Diff-Serv(8 bits)


Length in octets

Bit 0 Bit 31


IP packet




17=UDP

The second row is used for reassembling fragmentedIP packets, but fragmentation is quite rare,

so we will not look at these fields.

8-65


IP Version 4 Packet


(0100)


Flags(3 bits)



Diff-Serv(8 bits)


Length in octets

Bit 0 Bit 31


IP packet




17=UDP

The sender sets the time-to-live value (usually 64 to 128).Each router along the way decreases the value by one.

A router decreasing the value to zero discards the packet.It may send an ICMP error message.

The protocol field describes the message in the data field(1=ICMP, 2=TCP, 3=UDP, etc.)

The header checksum is used to find errors in the header.If a packet has an error, the router drops it.

There is no retransmission at the internet layer,so the internet layer is still unreliable.

8-66


IP Version 4 Packet

Source IP Address (32 bits)

Bit 0 Bit 31

Destination IP Address (32 bits)

PaddingOptions (if any)

Data FieldThe source and destination IP addressesAre 32 bits long, as you would expect.

Options can be added, but these are rare.

8-67


IP Version 6 Packet

Source IP Address (128 bits)

Bit 0 Bit 31

Hop Limit(8 bits)

Next Header(8 bits) Nameof next header

Payload Length(16 bits)


(0110)

Diff-Serv(8 bits)

Flow Label (20 bits)Marks a packet as part of a specific flow

Destination IP Address (128 bits)

Next Header or Payload (Data Field)

IP Version 6 is the emergingversion of the Internet protocol.

Has 128 bit addresses foran almost unlimited number of IP addresses.

Needed because of rapid growth in Asia.

Also needed because of the explodingnumber of mobile devices

The Transmission Control Protocol (TCP)

8-69

Figure 8-18: TCP Segment and UDP Datagram

TCP Segment

Window Size(16 bits)

Bit 0 Bit 31

Destination Port Number (16 bits)Source Port Number (16 bits)

Sequence Number (32 bits)

Acknowledgment Number (32 bits)

Urgent Pointer (16 bits)TCP Checksum (16 bits)


Reserved(6 bits)

Flag Fields(6 bits)

Flag fields are one-bit fields. They include SYN, ACK, FIN,and RST.

The source and destination port numbersspecify a particular application on the

source and destination multitasking computers(Discussed later)

Sequence numbers are 32 bits long.So are acknowledgment numbers.

8-70


TCP Segment


Bit 0 Bit 31






Reserved(6 bits)

Flag Fields(6 bits)

Flags are one-bit fields.If a flag’s value is 1, it is “set”.

If a flag’s value is 0, it is “not set.”TCP has six flags

If the TCP Checksum field’s value is correct,The receiving process sends back an acknowledgment.

8-71


TCP Segment


Bit 0 Bit 31






Reserved(6 bits)

Flag Fields(6 bits)

For flow control (to tell the other party to slow down),The sender places a small value in the Window Size field.

If the Window Size is small, the receiver will have to stop transmittingafter a few more segments (unless it gets a new acknowledgment

extending the number of segments it may send.)

8-72


TCP SegmentBit 0 Bit 31

PaddingOptions (if any)

Data Field

TCP segment headers can end with options.Unlike IPv4 options,

TCP options are very common.

If an option does not end at a 32-bit boundary,padding must be added.

The User Datagram Protocol (UDP)

8-74


UDP DatagramBit 0 Bit 31

Source Port Number (16 bits) Destination Port Number (16 bits)

UDP Length (16 bits) UDP Checksum (16 bits)

Data Field

UDP messages (datagrams) are very simple.Like TCP, UDP has 16-bit port numbers.

The UDP length field allows variable-length application messages.If the UDP checksum is correct, there is no acknowledgment.

If the UDP checksum is incorrect, the UDP datagram is dropped.

8-75

Figure 8-19: TCP Connection Openings and Closings

• TCP is a connection-oriented protocol

– Each connection has a formal opening process

– Each connection has a formal closing process

– During a connection, each TCP segment is acknowledged

• (Of course, pure acknowledgments are not acknowledged)

8-76


SYN

SYN/ACK

ACK

Normal Three-Way Opening

A SYN segment is a segment in which the SYN bit is set.One side sends a SYN segment requesting an opening.The other side sends a SYN/acknowledgment segment.

Originating side acknowledges the SYN/ACK.

8-77


FIN

ACK

FIN

ACK

Normal Four-Way Close

A FIN segment is a segment in which the FIN bit is set.Like both sides saying “good bye” to end a conversation.

8-78


RST

Abrupt Reset

An RST segment is a segment in which the RST bit is set.A single RST segment breaks a connection.

Like hanging up during a phone call.There is no acknowledgment.

Port Numbers and Sockets in TCP and UDP

8-80

TCP and UDP Port Numbers

• Computers are multitasking devices

– They run multiple applications at the same time

– On a server, a port number designates a specific applications

Server

HTTP WebserverApplication

SMTP E-MailApplications

Port 80 Port 25

8-81


• Major Applications Have Well-Known Port Numbers– 0 to 1023 for both TCP and UDP– HTTP is TCP Port 80– SMTP is TCP Port 25

Server

HTTP WebserverApplication

SMTP E-MailApplications

Port 80 Port 25

8-82


• Clients Use Ephemeral Port Numbers– 1024 to 4999 for Windows Client PCs– A client has a separate port number for each connection

to a program on a server

Client

Port 4400Port 3270

WebserverApplication

on Webserver

E-MailApplication

on MailServer

8-83

Figure 8-20: Use of TCP (and UDP) Port Numbers

Client 60.171.18.22

Webserver1.33.17.13

Port 80

SMTP Server123.30.17.120

Port 25

A socket is anIP address, a colon, and a port number.

1.33.17.3:80123.30.17.120:25

128.171.17.13:2849

It represents a specific application (Port number)on a specific server (IP address)

Or a specific connection on a client.

Client PC128.171.17.13

Port 2849

8-84


Client60.171.18.22

Webserver1.33.17.13

Port 80

Source: 60.171.18.22:2707Destination: 1.33.17.13:80


Port 25

This shows sockets for a clientpacket sent to a webserver application

on a webserver

8-85


Client60.171.18.22

Webserver1.33.17.13

Port 80




Port 25

Sockets intwo-way

transmission

8-86


Client60.171.18.22

Webserver1.33.17.13

Port 80





Port 25Clients use a different ephemeralport number for different connections

Layer 3 Switches

8-88

Figure 8-21: Layer 3 Switches and Routers in Site Networks

Router

Ethernet WorkgroupSwitch

ToOtherSites

Layer 3Switch

L3

L3

Layer 3 switches arerouters.

Layer 3 switches arefaster and cheaper tobuy than traditionalrouters.

However, they areusually limited infunctionality.

They also areexpensive to manage.

They are typicallyused between

Figure 8-21: Layer 3 Switches and Routers in Site Internets

Ethernet WorkgroupSwitch

Layer 3Switch

Usually too expensive to replace workgroup switches.Usually too limited in functionality to replace border routers.

Replaces core switches in the middle.

Documents

TCP/IP Internetworking Chapter 8. 8-2 Recap Single Networks (Subnets) –Chapters 4 and 5 covered single LANs –Chapters 6 and 7 covered residential Internet