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Page 1: 11- IP Network Layer4-1. Network Layer4-2 The Internet Network layer forwarding table Host, router network layer functions: Routing protocols path selection

11- IP

Network Layer 4-1

Page 2: 11- IP Network Layer4-1. Network Layer4-2 The Internet Network layer forwarding table Host, router network layer functions: Routing protocols path selection

Network Layer 4-2

The Internet Network layer

forwardingtable

Host, router network layer functions:

Routing protocols•path selection•RIP, OSPF, BGP

IP protocol•addressing conventions•datagram format•packet handling conventions

ICMP protocol•error reporting•router “signaling”

Transport layer: TCP, UDP

Link layer

physical layer

Networklayer

Page 3: 11- IP Network Layer4-1. Network Layer4-2 The Internet Network layer forwarding table Host, router network layer functions: Routing protocols path selection

Network Layer 4-3

IP datagram format

ver length

32 bits

data (variable length,typically a TCP

or UDP segment)

16-bit identifier

header checksum

time tolive

32 bit source IP address

IP protocol versionnumber

header length (bytes)

max numberremaining hops

(decremented at each router)

forfragmentation/reassembly

total datagramlength (bytes)

upper layer protocolto deliver payload to

head.len

type ofservice

“type” of data flgsfragment

offsetupper layer

32 bit destination IP address

Options (if any) E.g. timestamp,record routetaken, specifylist of routers to visit, don’t fragment bit.

Page 4: 11- IP Network Layer4-1. Network Layer4-2 The Internet Network layer forwarding table Host, router network layer functions: Routing protocols path selection

Network Layer 4-4

IP Fragmentation & Reassembly network links have MTU

(max.transfer size) - largest possible link-level frame. different link types,

different MTUs large IP datagram divided

(“fragmented”) within net one datagram becomes

several datagrams “reassembled” only at

final destination IP header bits used to

identify, order related fragments

fragmentation: in: one large datagramout: 3 smaller datagrams

reassembly

Page 5: 11- IP Network Layer4-1. Network Layer4-2 The Internet Network layer forwarding table Host, router network layer functions: Routing protocols path selection

Network Layer 4-5

IP Fragmentation and Reassembly

Example 4000 byte

datagram MTU = 1500

bytes

• length – number of bytes• ID – 16 bit identifier• fragflag

• 0 => no more fragments• 1 => more fragments to follow

• offset – (byte number)/8

Page 6: 11- IP Network Layer4-1. Network Layer4-2 The Internet Network layer forwarding table Host, router network layer functions: Routing protocols path selection

Network Layer 4-6

IP Addressing: introduction IP address: 32-bit

identifier for host, router interface

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

multiple interfaces host 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

Page 7: 11- IP Network Layer4-1. Network Layer4-2 The Internet Network layer forwarding table Host, router network layer functions: Routing protocols path selection

Network Layer 4-7

Subnets IP address:

subnet part/prefix (high order bits)

host part (low order bits)

What’s a subnet ? device interfaces

with same subnet 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 subnets

LAN

Page 8: 11- IP Network Layer4-1. Network Layer4-2 The Internet Network layer forwarding table Host, router network layer functions: Routing protocols path selection

Network Layer 4-8

Subnets 223.1.1.0/24223.1.2.0/24

223.1.3.0/24

Recipe To determine the

subnets, detach each interface from its host or router, creating islands of isolated networks. Each isolated network is called a subnet. Subnet mask: /24

Page 9: 11- IP Network Layer4-1. Network Layer4-2 The Internet Network layer forwarding table Host, router network layer functions: Routing protocols path selection

Network Layer 4-9

SubnetsHow many? 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

Page 10: 11- IP Network Layer4-1. Network Layer4-2 The Internet Network layer forwarding table Host, router network layer functions: Routing protocols path selection

“Classful” addressing

Network Layer 4-10

Page 11: 11- IP Network Layer4-1. Network Layer4-2 The Internet Network layer forwarding table Host, router network layer functions: Routing protocols path selection

Network Layer 4-11

IP addressing: CIDR

CIDR: Classless InterDomain Routing subnet portion of address of arbitrary length address format: a.b.c.d/x, where x is # bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

200.23.16.0/23

Page 12: 11- IP Network Layer4-1. Network Layer4-2 The Internet Network layer forwarding table Host, router network layer functions: Routing protocols path selection

Network Layer 4-12

IP addresses: how to get one?

Q: How does host get IP address?

hard-coded by system admin in a file DHCP: Dynamic Host Configuration Protocol:

dynamically get address from a server “plug-and-play”

(more in next chapter)

Page 13: 11- IP Network Layer4-1. Network Layer4-2 The Internet Network layer forwarding table Host, router network layer functions: Routing protocols path selection

Network Layer 4-13

IP addresses: how to get one?

Q: How does network get subnet 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

Page 14: 11- IP Network Layer4-1. Network Layer4-2 The Internet Network layer forwarding table Host, router network layer functions: Routing protocols path selection

Network Layer 4-14

Hierarchical addressing: route aggregation

“Send me anythingwith addresses beginning 200.23.16.0/20”

200.23.16.0/23

200.23.18.0/23

200.23.30.0/23

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us“Send me anythingwith addresses beginning 199.31.0.0/16”

200.23.20.0/23Organization 2

...

...

Hierarchical addressing allows efficient advertisement of routing information:

Page 15: 11- IP Network Layer4-1. Network Layer4-2 The Internet Network layer forwarding table Host, router network layer functions: Routing protocols path selection

Network Layer 4-15

Hierarchical addressing: more specific routes

ISPs-R-Us has a more specific route to Organization 1

“Send me anythingwith addresses beginning 200.23.16.0/20”

200.23.16.0/23

200.23.18.0/23

200.23.30.0/23

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us“Send me anythingwith addresses beginning 199.31.0.0/16or 200.23.18.0/23”

200.23.20.0/23Organization 2

...

...

Page 16: 11- IP Network Layer4-1. Network Layer4-2 The Internet Network layer forwarding table Host, router network layer functions: Routing protocols path selection

Network Layer 4-16

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


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