Computer networks routing fundamentals and subnets

Computer networks routing fundamentals and subnets

Routing Fundamentals and Subnets

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Routing overview Routing versus switching Routed versus routing Path determination Routing tables

Routing algorithms and

metrics

IGP and EGP Link state and distance vector

Routing protocols

10.3 The Mechanics of Subnetting

Classes of network IP addresses

Introduction to and reason for

subnetting Establishing the subnet mask address

Applying the subnet mask Subnetting Class A and B networks

Calculating the resident subnetwork through ANDing

10.1 Routed Protocol

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Routable and routed protocols

IP / IPX NetBEUI 2?

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192.168.10.250 192.168.10.251 192.168.10.252 192.168.10.253

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IP as a routed protocol

Hierarchical network-addressing scheme

IP is a connectionless, unreliable, best-effort delivery

protocol

The term connectionless means that no dedicated

circuit connection is established prior to transmission

as there is when placing a telephone call

The terms unreliable and best-effort do not imply that the system is unreliable and does not work well, but that IP does not verity that the data reached its

destination This function is handled by the upper layer protocols

IP determines the most efficient route for data based

on the routing protocol

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IP as a routed protocol (conI.)

Packet propagation and switching within a

Frame | Network ,_, | Frame

Ỷ Physical Header | Header “ Trailer

The transport layer again segments, sequences and adds error checking to the email message The network layer source and destination addresses are added to the datagram The ARP cache provides the MAC address for the destination IP address,

so the Ethernet frame is added with the source and destination addresses

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Packet propagation and switching within a router (cont.)

Destination Source Source Destination

DD:EC:BC:AB:04:AC | FE:ED:F9:44:45:66 ' 198 150 11 34! 198 150.11 163' Email CRC-32

Packet propagation and switching within a router (cont.)

Client (Bill Smith) Toolbar: Maximize 198.150.11.34 198.150.11.163

The router picks up the frame which was addressed to its MAC address and

strips off the Ethernet frame

Packet propagation and switching within a router (cont.)

498.1504145 — DD:EC:BC:AB:04:AC DD:EC:BC:43:7B:34 198.150.11.165

198.150 11.163 IP Address 255.255.255.224 Subnet mask 198.150 11.160 Result

The router applies the subnet mask to the destination address

The router then compares the result to its router table The table shows that to get to network 198.150.11.160 the packet must be forwarded out the serial (198.150.11.65) port on the router

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Packet propagation and switching within a router (cont.)

Client (Bill Smith) Toolbar: Maximize

198.150.11.34 198.150.11.163 FE:ED:F9:44:45:66 FE:ED:F9:EF:ED:EC

198.150.1165 198.150.11.66

.150.11.33 198.150.11.161 198.150.11.45 DD:EC:BC:AB:04:AC DD:EC:BC:43:7B:34 198.150.11.165

PPP PPP 198.150.11.34) 198.150.11.163 | Email | CRC-32

Network

Header

Data Link PPP | Network Ppp

Physical Header| Header Trailer

The request is encapsulated for serial transmission and sent to the next router

Packet propagation and switching within a router (cont.)

Client (Bill Smith) Toolbar: Maximize 198.150.11.34 198.150.11.163 FE:ED:F9:44:45:66 FE:ED:F9:EF:ED:EC

198.150.11.65 198.150.1

198.150.11.33 198.150.11.161

198.150.11.45 DD:EC:BC:AB:04:AC DD:EC:BC:43:7B:34 198.150.11.165 FE:ED:F9:FE:44:AF FE:ED:F9:89:54:23

Packet propagation and switching within a router (cont.)

Client (Bill Smith)

The router applies the subnet mask to the destination address The router then compares the result to its router table The table shows that to get to network 198.150.11.160 the packet must be forwarded out the Ethernet

(198.150.11.161) port on the router

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Packet propagation and switching within a router (cont.)

198.1 198.150.11.45 DD:EC:BC:AB:04:AC DD:EC:BC:43:7B:34 198.150.11.165

Frame Header Network Header Data Frame

Trailer

FE:ED:F9:EF:ED:EC ' DD:EC:BC:43:7B:34 198.150.11 34 198 150 11 163 ': Email ': CRC-32

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Packet propagation and switching within a router (cont.)

The recieving computer de-encapsulates the data packet and processes the data

This involves the transport layer reassembling the data packets in the proper order

and checking for errors

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Packet propagation and switching within a router (cont.)

header and trailer a address with routing table the new CRC

destination IP data packet in

address from the appropnate

frame header

Ỷ

Forward the new frame

Internet Protocol (IP)

2 types of delivery services are connectionless and connection-oriented

They provide the actual end-to-end delivery of data in an inter-network Most network services use a connectionless delivery system

Connectionless service (packet switched)

Packets may take different paths to get through the network (possibly

arrive out of order), but are reassembled after arriving at the

estination

The destination is not contacted before a packet is sent

Connection-oriented (circuit switched)

A connection is established between the sender and the recipient before any data is transferred

All packets travel sequentially across the same physical or virtual circuit

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Anatomy of an IP packet

The header fields are the information that is provided to the upper layer protocols defining the data in the packet

Version — The version of IP currently used; different version > reject the packets

IP header length (HLEN) — The datagram header length in 32-bit words

Type-of-service (TOS) — The level of importance that has been assigned by a particular upper-layer protocol

Total length — The length of the entire packet in bytes, including data and header

To get the length of the data payload subtract the HLEN from the total length

Identification — Integer, the sequence number

Flags — information of fragmentation One bit specifies whether the packet can be fragmented, and the other 2-bit specifies whether the packet is the last fragment in

a series of fragmented packets

Source IP Address Destination IP Address

_IP Options (if any) | Padding

These are the header fields in an IP packet header All field lengths

are fixed except for IP options and the padding fields

Anatomy of an IP packet (cont.)

Fragment offset — Used to help piece together datagram fragments, 13 bits This field allows

the previous field to end on a 16-bit boundary

Time-to-live (TTL) — A field that specifies the number of hops a packet may travel This number

is decreased by one as the packet travels through a router When the counter reaches zero the packet is discarded This prevents packets from looping endlessly

Protocol — indicates which upper-layer protocol, such as TCP or UDP

Header checksum — helps ensure IP header integrity

Source address — specifies the sending node IP address

Destination address — specifies the receiving node IP address

Options — allows IP to support various options, such as security, variable length

Fadaing — extra zeros are added to this field to ensure that the IP header is always a multiple of its

Data — contains upper-layer information, variable length up to 64 Kb

Time to Live Protocol Header Checksum

These are the header fields in an IP packet header All field lengths

are fixed except for IP options and the padding fields

10.2 IP Routing Protocols

Routing overview

Routing is an OS]! Layer 3 function

Routing is the process of finding the most efficient path from one device

to another

The primary device that performs the routing process is the router

The following are the two key functions of a router:

Routers must maintain routing tables and make sure other routers

know of changes in the network topology This function is performed using a routing protocol to communicate network information with other

routers

When packets arrive at an interface, the router must use the routing table to determine where to send them The router switches the packets to the appropriate interface, adds the necessary framing

Information for the interface, and then transmits the frame

The encapsulation and de-encapsulation process occurs each time a packet transfers through a router

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Routing overview (cont )

Delay Reliability Load Cost

The network layer is responsible for routing packets through a network

Routing versus switching

The primary difference

Switching occurs at Layer 2, the data link layer

Routing occurs at Layer 3, network layer

Routing and switching use different information in the process of moving data from source to destination

Telephone systems, local and long distance calls

i

Layer 2 switching based addres IN Layer 2 switching Se

sai ain based on MAC address

Routing versus switching (cont )

Routing Table Leamed Network Address Hop Interface

Routing versus switching (cont )

as the actual addresses are too long to fit in the graphic)

The routing tables also track how the route was learned (in this case either directly connected [C] or learned by RIP [R]), the network IP address for reachable networks, the hop count or distance to those networks, and the interface the data must be sent out to get to the destination network

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Routing versus switching (cont )

Switched networks do not block broadcasts

Routers block broadcasts, routers also provide a higher level of security and bandwidth control than

Security Higher Lower

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Houted versus routing

Protocols used at the network layer that transfer data from one

host to another across a router ES

are called routed or routable

rotocols Routed protocols ol `

transport data across a network RIP | "-

Routing protocols allow routers to = ———+—

choose the best path for data 'DECNeI [7 from source to destination _AppleTalk | vy |

Routers use routing protocols to IBaumUNS | Ý |

share routing information In other words, routing protocols enable routers to route routed protocols

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Routed versus routing (conI.)

A routed protocol functions include the following:

Includes any network protocol suite that provides enough information in its network layer address to allow a router to forward it to the next device and ultimately to its destination

Defines the format and use of the fields within a packet

A routing protocol functions includes the following:

Provides processes for sharing route information

Allows routers to communicate with other routers to

update and maintain the routing tables

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Routed versus routing (conI.)

Routed versus routing (conI.)

Network Destination Exit

Protocol Network Interface

Connected |10.120.2.0 E0 RIP 172.16.2.0 S0

routers to determine paths

and maintaining routing tables

After the path is determined

a router can route a routed protocol

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Path determination

Path determination enables a router to compare the destination address to the available routes in its

routing table, and to select the best path

The routers learn of these available routes through

static routing or dynamic routing

Routes configured manually by the network administrator are static routes

Routes learned by others routers using a routing protocol are dynamic routes

Routers can make decisions based on the load,

bandwidth, delay, cost, and reliability of a network

link

Each router that the packet encounters along the way

is called a hop

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Path determination (cont.)

———

: Apply ae af Forward the date

Receive frame Remove routing table entry ——œ đc ih ee

on interface frame header on destination IP routing table entry

4 Compare masked Ecapsulale the

Y An Sư = destination IP data packet in the

Pi mation address with entry appropriate frame

a“ “Yes | address from network address

& SE uter? | ¥

Get first A ich? frame

rauting table _ od y entry = “No |

¥ Ayes] =<

Discard data | „fe there | Na | “1s thera | End | —

Routing tables

Routers use routing protocols to build and maintain routing tables that contain route information

This information varies depending on the routing protocol used

Routers keep track of important information in their routing tables, including the following:

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Protocol type — The type of routing protocol that created the routing table entry Destination/next-hop associations — These associations tell a router that a particular destination is either directly connected to the router, or that it can be reached using another router called the “next-hop” on the way to the final

destination

When a router receives an incoming packet, it checks the destination address and attempts to match this address with a routing table entry

Routing metric — Different routing protocols use different routing metrics

Routing metrics are used to determine the desirability of a route

Routing Information Protocol (RIP) uses hop count as its only routing metric

Interior Gateway Routing Protocol (IGRP) uses a combination of bandwidth, load, delay, and reliability metrics to create a composite metric value

Outbound interfaces — The interface that the data must be sent out on, in order

to reach the final destination

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