Lecture 15: Addressing and Routing Architectureshervin/courses/ceg4185/lectures/...Prof. Shervin Shirmohammadi CEG 4185 15-8 ClassfulAddressing • Outdated form of addressing offers

Prof. Shervin Shirmohammadi

CEG 4185

15-1

Lecture 15:

Addressing and Routing Architecture


SITE, University of Ottawa


CEG 4185

15-2

Addressing & Routing

•Addressingis assigning identifiersto devices.

These identifiers can be local or global, private or

public, temporary or persistent.

•Routingconsists of learning about the reachability

within and between networks and applying this

reachability to forward packets in the network.

•Together, they form a complete picture of network

connectivity.


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15-3

Addressing Fundamentals

•IP Addressing uses a combination of Address

Identifier and Mask

–The mask is used to separate the address into a network

and host function.

–This is very important in the distinction between local

and remote parts of the network.

–E.g.:

Which means 129.99.30.4 is on subnet 129.99.16.0

129.99.30.4

255.255.240.0

Address Identifier

Mask

XOR

==

129.99.16.0

Subnet


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Type of Addresses

•Localor Global

–Local communication addresses like link-layer (MAC Address).

–Not advertised outside of the local network: there's no point since

there is no link-layer connectivity between non-local devices.

–Global addresses are required for devices outside of the local

broadcast region like IP addresses.

•Privateor Public

–Both are global addresses, but private addresses are not advertised

and forwarded (on purpose) while public addresses are.

•Temporaryof Persistent

–Temporary are usually assigned using DHCP while persistent

addresses are assigned either manually or are hardcoded (like

Ethernet address “carved”into an Ethernet network card.


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Local vs. Remote Network

Network

.0.1

.0.2

.0.3

Other Network

136.178.0.1

129.99.0.100

136.178.0.100

Devices on the same subnet are

directly connected and therefore, for

IP, address resolution is done at

different layer (MAC) than that done at

the routing layer (IP).

In communicating to devices on

other networks there must be a

router connecting the networks

136.178.0.0/16

129.99.0.0/16


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Explicit Routing

Company A

129.99.0.0

255.255.0.0

Company B

129.99.10.0

255.255.255.0

ISP X

ISP Y

ISP Z

Routing Table

129.99.0.0/255.255.0.0

129.99.10.0/255.255.255.0

Internet

Route to

129.99.0.0

Route to

129.99.10.0

Packets to

129.99.10.0 are

routed here.


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Addressing Mechanisms

•Classfuladdressing

–older style of addressing

•Subnetting

–A better way to distribute addresses

•Variable-length subnetting

–Even more refined than subnetting

•Supernettingand Classless interdomainrouting (CIDR).

–An efficient way to advertise addresses, and currently used on

the Internet.

•Private addressingand Network Address Translation

(NAT).

–A way to re-use certain IP addresses without collision with

the rest of Internet


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ClassfulAddressing

•Outdated form of addressing offers a simplistic solution

for addressing schemes.

•Based on pre-determined mask lengths where:

–Class A = Mask 255.0.0.0 (127 Networks & over 16M

Addresses/Network), First Octet Range 1-127

–Class B = Mask 255.255.0.0 (16K Networks & 64K


–Class C = Mask 255. 255. 255.0 (2M Networks & 254


–Class D = Multicast address

–Class E is reserved.

0 10

110


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Limits to ClassfulAddressing

•Very few

Class A and B addresses, and all have

already been allocated.

–That leaves class C to allocate new addresses

•Many networks require more addressesthan

class C but fewer addressesthan B offers.

–On the other hand many organizations with A or B

cannot use all of the networks offered by class A or

B.

•That has led to variable-length subnets


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Subnetting

•Allows a classfulnetwork address to be segmented into

smaller sectionsby using part of the device address to

create another level of hierarchy.

•Basically it takes address space away from the devices

and gives it to the network.

•Useful forInternaladdressing and routing

–Allows you to assign subnets to specific buildings, or specific

groups, hence localizing traffic and simplifying routing.

•Has no effect on external routing

–The hierarchy is not revealed to the outside world.

129.99.30.4

255.255.240.0

Address Identifier

Mask

XOR

==

129.99.16.0

Subnet


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Subnettingfor Class B Network

2-Bit Mask

255.255.192.0

3-Bit Mask

255.255.224.0

4-Bit Mask

255.255.240. 0

5-Bit Mask

255.255.248. 0

6-Bit Mask

255.255.252. 0

7-Bit Mask

255.255.254. 0

8-Bit Mask

255.255.255. 0

Class B

255.255.0.0

1 Network

64K Devices

3 Subnets

16382 Devices/Subnet

7 Subnets

8190 Devices/Subnet

15 Subnets

4094 Devices/Subnet

31 Subnets

2046 Devices/Subnet

63 Subnets

1022 Devices/Subnet

127 Subnets

510 Devices/Subnet

255 Subnets

254 Devices/Subnet

Notice that all zerosare not allowed as either subnet part or as host

part. Also, all onesare not allowed for the host part.


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Example

•A company has bought IP class address 136.178.0.0. It has 14

departments and it wants to give each its own subnet. What will

be each of the subnets, and their subnet mask?

•This is a class “B”address so we subnet into the 3rdoctet. To

have 14 subnets, we require 2^4 = 16 subnet divisions, so we

play with the first 4 bits in the third octet. The mask will be

255.255.240.0, and subnets are:

1. 136.1

78.0

.0 1

0001000.1

0110010.0

000 0

000.0

0000000

2. 136.1

78.1

6.010001000.10110010.0001 0000.00000000

3. 136.1

78.3

2.010001000.10110010.0010 0000.00000000

4.136.1

78.4

8.010001000.10110010.0011 0000.00000000

5. 136.1

78.6

4.010001000.10110010.0100 0000.00000000

6. 136.1

78.8

0.010001000.10110010.0101 0000.00000000

7. 136.1

78.9

6.010001000.10110010.0110 0000.00000000

8. 136.1

78.1

12.010001000.10110010.0111 0000.00000000

9. 136.1

78.1

28.010001000.10110010.1000 0000.00000000

10. 136.1

78.1

44.010001000.10110010.1001 0000.00000000

11. 136.1

78.1

60.010001000.10110010.1010 0000.00000000

12. 136.1

78.1

76.010001000.10110010.1011 0000.00000000

13. 136.1

78.1

92.010001000.10110010.1100 0000.00000000

14. 136.1

78.2

08.010001000.10110010.1101 0000.00000000

15. 136.1

78.2

24.010001000.10110010.1110 0000.00000000

16. 136.1

78.2

40.010001000.10110010.1111 0000.00000000


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Variable-length Subnetting

•Subnettingdivides the network into a number of equal-sized

subnets which is often inefficient.

•Variable-length subnettingis subnettingin which non-equal or

variable lengthsubnets are used.

•E.g., an organization with Class B address 137.178.0.0 has a

number of workgroups divided as shown below:

5730

43

Tota

l

10-40

22

Support

150

1R&D

35-90 (1350)

15

Sales

200

1Administration

1950

1Marketing

400 (1200)

3Engineering

Siz

e/G

roup (D

evic

es)

Gro

ups

Work

gro

up


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Classfuland SubnettingSolution

•Classfulsolution gives us 65,534 devices. That’s

enough devices, but putting all departments (i.e., 5730

devices) in the same “subnet”is neither scalable nor

manageable.

•Subnettingsolution:

–let’s have 1 subnet per group. We have 43 groups, so we

need 2^6 –1 = 63 subnets, which means we can use 6 bits for

the subnet part, and the remaining 10 bits for hosts.

–But 10 bits for hosts part gives us 2^10 –2 = 1022 devices,

and marketing has 1950 devices.

–What to do?

•Use variable length subnetting.


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Variable-length SubnettingSolution

•We can resolve this by using a combination of 4-bit and 8-bit masks.

•4-bit mask 255.255.240.0 gives us 15 subnets each with 4096

devices. That’s enough for Engineering and Marketing. We use the

first five subnets as follows:

–Engineering (3 subnets) -> 136.178.16.0, 136.178.32.0, 136.178.48.0

–Marketing (1subnet) -> 136.178.64.0

–Administration (1 subnet) -> 136.178.80.0

•8-bit mask 255.255.255.0 gives us 255 subnets and 254 devices.

That’s fine for Sales, R&D, & Support.

–We use as many as the remaining 10 subnets as needed, breaking them into

sub-subnets. For example, the next subnet, 136.178.96.0 can be broken into

another 15 subnets from 136.178.97.0 to 136.178.111.0. All these15 will go to

Sales.

–We still need another 23 subnets: 1 for R&D and 22 for Support. For these, we

break the next two subnets, 136.178.112.0 and 136.178.128.0

Why Admin?


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Supernetting

•Supernettingis the concept of aggregating network addresses by

changing the network mask to decrease the number of bits

recognized as the network part.

•Millions of Class C addresses can be allocated in lieu of Class

A & B.

–The result is that too many Class C address groups need to be allocated

to an organization and advertised among all the Internet routers.

–The number of routes would grow exponentially such that some

experts had predicted that the Internet would collapse by 1995.

–Obviously this did not happen, since supernettingwas invented.

•Say a company needs to support 10,000 devices.

–A class C address supports up to 254 devices, so 40 class C networks

are needed.

–How are we to advertise these 40 class C addresses?


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SupernettingTechnique

•If we take a set of 16 contiguousaddresses from a Class C address like

192.92.240.0 we can see that the first 4 digits of the subnet octet do not

change.

•This range of values can be represented as

192.92.240.0 with a subnet mask of

255.255.240.0 where the last 4 bits

in the third octet are ignored.

•This then can be used to advertise a group of addresses as 192.92.240.0/20

which means addresses from 192.92.240.0 -> 192.92.255.0


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Classless InterDomainRouting (CIDR)

•The concept of supernettingsuggested that indeed we do not need class

boundaries, since each “group”can advertise its own subnet mask too.

•This in effect lead to classless ClasslessInterDomainRouting (CIDR).

•Addresses must be assigned in contiguousblocks following logical

topology.

•The number of addresses in a CIDR block are powers of 2.

•Network Prefix can be anything, and need not be a power of 2. Itis

transmitted along with address

•Used in conjunction with classless routing protocols (e. g. EIGRP, OSPF)

•E.g.:

–192.92.240/22 advertises 4 networks: 240, 241, 242, and 243

–192.92.240/23 advertises 2 networks: 240 and 241

–192.92.240/24 (this is the natural mask for class C) advertises 1 network: 240

–192.92.240/21 advertises 8 networks: 240 to 247

–200.1.128.0/17 is equivalent to a range of 27, or 128, networks from

200.1.128.0 -> 200.1.255.0


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Classfulvs. CIDR

•Classfulrouter must advertise all 4 nets

•Classless router only advertises one

172.16.0.0

172.16.0.0

172.17.0.0

172.17.0.0

172.18.0.0

172.18.0.0

172.19.0.0

172.19.0.0

Rtr1

I’m router 1 and

I know how to get to

networks:

172.16.0.0

172.17.0.0

172.18.0.0

172.19.0.0

172.16.0.0

172.16.0.0

172.17.0.0

172.17.0.0

172.18.0.0

172.18.0.0

172.19.0.0

172.19.0.0

Rtr1

I’m router 1 and I know

how to get to networks:

172.16.0.0/14

172.16

172.16

10101100 000100

10101100 0001000000

172.17

172.17

10101100 000100

10101100 0001000101

172.18

172.18

10101100 000100

10101100 0001001010

172.19

172.19

10101100 000100

10101100 0001001111

14 bits

14 bits


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Private Addresses and NATs

•Private IP Addressesare reserved addresses that can’t be

forwarded to the Internet

10.0.0.0 -> 10.255.255.255 (10/8 prefix)

172.16.0.0 -> 172.31.255.255 (172.16/12 prefix)

192.168.0.0 -> 192.168.255.255 (192.168/16 prefix)

•Pros:

–Makes changing ISP easier

–Increases security

•Cons:

–Outsourcing management may be difficult

–Mergers may require renumbering

•Network Address Translation: translates private addresses <->

public addresses

–A binding is created between the addresses that lasts a period of time.

•Can be implemented in Router, Firewall, or Specialized device.


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Routing

•Staticrouting

–The reachability is entered manually to the router. Method we

commonly use for our small networking labs.

•Dynamicrouting

–More typical of a real network. Typical routing protocols are:

RIP/RIPv2, OSPF, and BGP4

–Destination is determined by looking at the network portion of the

packets destination address and choose the best destination (one

with the more explicit route. i.e. the more specific).

•We now consider a routing mechanism that consists of:

–Establishing routing flows

–Identifying and classifying routing boundaries

–Manipulating routing flows .

•This will be based on the flow analysis process that leverages the flow

specification and flow map discussed in lecture 9.


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Establishing Routing Flows

•Segment the network into functional areasand workgroups.

•Identify boundaries between these areas.

•Form relationships between boundaries and routing flows.

•A functional areaconsists of groups within the system that

share a similar function.

–These may consist of users (workgroups), applications, devices, or

combinations of these and they may share similar jobs, locations,

functions within the network (backbone routing).

•Workgroupsare groups of users that have common locations,

applications, and requirements, or that belong to the same

organization.


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Example of Workgroups & FAs

Bldg A

Scientists

FA1

Bldg B

Management

FA4

WG1

WG2FAb1

FAb3

FA2

FA3

Scientists

Accounting

Bldg C

Routers


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Routing Boundaries

•These are physical or logical separations of a network

based on requirements or administration of the

network.

•Physical Boundariescan de identified by isolated

LANs, DMZs, physical interfaces on network

equipment, physical security.

•Logical Boundariescan be identified by the FAs, WGs,

administrative domains (Autonomous Systems AS),

and routing management domains.


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Hard Boundaries

•These boundaries are routing boundaries in which EGPsare

predominantly used:

–Exterior Gateway Protocols(EGPs) communicate between AS’s or AS

and external network. DMZsand interfaces to ISP.

–Interior Gateway Protocols(IGPs) communicate within an AS.

Your AS/Administrative Domain

Internet

DMZ

Hard Boundary


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Soft Boundaries

•Typically found within a single AS and are usually placed

at the junction of FAsand WGs

FA1

WG2

WG1

WG1

WG1

WG1

WG1

WG1

FA2

FA3

FA4

Your AS / Administrative Domain


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Internet Routing

•The Internet uses hierarchical routing

•The Internet is split into AS’s

–AS corresponds to an administrative domain

–Assign each AS a 16-bit number

–Examples: University, company, backbone network

•Stanford (32), Sprint (1239), MCI Worldcom(17373)

•Within an AS, the administrator chooses an Interior

Gateway Protocol (IGP)

–Examples of IGPs: RIP (RFC 1058), OSPF (RFC 1247)

–Between AS’s, the Internet uses an Exterior Gateway Protocol

•AS’s today use the Border Gateway Protocol, BGP-4 (RFC 1771)


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Why different Intra-and Inter-AS routing?

•Policy:

–Inter-AS: admin wants control over how its traffic is routed

•who routes through its net.

–Intra-AS: single admin, so no policy decisions needed

•Scale:

–hierarchical routing saves table size, update traffic

•Performance:

–Intra-AS: can focus on performance

–Inter-AS: policy may dominate over performance


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•Routing Flows are flows of routing information passed

between FAsand ASs.

•These are important

for the architecture

and design because

routing flows can be

manipulated at routing

boundaries.Boundaries & Routing Flows

FA 1

FAb1

FAb2

FA4

FA5

FA3

External

Networks

FA2

Routing Flows

AS

Hard Boundary


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Manipulating Routing Flows

•Controlling routing flow in a network is vital to the proper operation

and performance of the network.

–This involves determining the proper combinationof addressingand routing.

•Techniques:

–Default Routing

–Route Filtering

–Route Aggregation

–Policies & Policy Enforcement Points

•Default Routeis the route used when there is no other route. Generally

the route with the highest capacity to the network.

•Route Filteringis a technique to hide networks from the rest of the

AS. Implemented as a rule (if IPPacketDest= 1.1.1.1 then

DropPacket)

•Route Aggregationis a technique to exchange routing between AS’s

•Policiesallow AS to accept or deny traffic,


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Addressing Strategies

•When addressing, we need to keep in mind the future

scaling requirements.

Area of Network

Addressing Scheme

Supernetting

(CIDR)

Natural Class

Subnetting

Variable-Length

Subnetting

Enterprise

Wide

Functional

Areas

Work

Groups

Networks

Hosts


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Example of Variable-length Subnetting

ISP

Router

ISP

WG1

WG2

WG3

WG4

WG5

Hub

Router

AS

•Hub router can interconnect up to 10 networks.

•WG routers can support 4 networks each with 10 to 20 devices

•CIDR block 192.92.240.0/20


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Solution

ISP

Router

ISP

192.92.241.32

192.92.241.64

192.92.241.96

Hub

Router

AS

192.92.242.32

192.92.242.64

192.92.242.96

192.92.243.32

192.92.243.64

192.92.243.96

192.92.244.32

192.92.244.64

192.92.244.96

192.92.245.32

192.92.245.64

192.92.245.96

/27

6 subnets

30 devices/subnet

/30

63 subnets

2 devices/subnet

192.92.240.0 .4 .8 .12

.16

.20

Documents

Lecture 15: Addressing and Routing Architectureshervin/courses/ceg4185/lectures/...Prof. Shervin Shirmohammadi CEG 4185 15-8 ClassfulAddressing • Outdated form of addressing offers