csis 4823 data communications networking – ip addressing review

CSIS 4823Data Communications

Networking – IP Addressing Review

Mr. Mark Welton

What is an IP address?

IPv4 address are 32-bit numbers represented in dotted decimal notation of 8 bit segments

00001010.00001000.01100100.00011000 10.8.100.24 So why 8 bit segments?

We started with a classful system (Class A,B,C,etc) Each class is created by 8-bits of the binary IP 8-bit processing systems where easier and

cheaper to build (RFC 791 published in 1981)

Somewhat Useless Trivia, Maybe?

11000000 10101000 00000000 00000000

Class A Class B Class C

We are accustomed to the decimal system a base 10 system

The number 12410 is 100+20+4 or◦ 1x102+2x101+4x100

◦ 1x100+2x10+4x1

A Fast Binary Review

The binary number system is a base 2 system

011111002 is 0x27+1x26+1x25+1x24+1x23+1x22+0x21+0x20

0x128+1x64+1x32+1x16+1x8+1x4+0x2+0x1

64+32+16+8+4 or 12410


So each octet (8-bit binary number) goes from◦ 000000002 – 1111111112

So what is the value of 1111111112


128+64+32+16+8+4+2+1 = 25510 So what is the hexadecimal value? 8+4+2+1 = 15 or F16


11111111128 64 32 16 8 4 2 1

27 26 25 24 23 22 21 20

IP address allocation is rarely done properly First mistake I see is people not understand

what I just covered Second mistake I see is not understanding

public vs private vs reserved IP addresses Third mistake I see is not understanding

how to take a large prefix and break it down to usable network prefixes that allow for growth

Fourth mistake I see is people not understanding why we do it

IP Address Allocation

We divide the IP space to create segments that makes sense to us

Segmentation = routing Each IP address allocation is a L2 network

which needs a router to move to the next network

The better we do this the easier routing and ACLs are to do

The easier the network is to troubleshoot


RFC 1918 “Address Allocation for Private Internets”

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) These are the IP address spaces that can be

used internally in an enterprise

Public Versus Private IP Space

“link local” block◦ 169.254.0.0 – 169.254.255.255 (169.254.0.0/16)◦ To be used when a device can not get an IP address through

DHCP reserves lowest Class B

◦ 128.0.0.0 -128.0.255.255 (128.0.0.0/16)◦ Not able to be used under old class system but can be assigned

to someone now Also defines loop back space (RFC 1700)

◦ 127.0.0.0 – 127.255.255.255 (127.0.0.0/8)◦ Used for a machine to communicate internally

Also defines multicast address space (RFC 5771)◦ 224.0.0.0 – 239.255.255.255 (224.0.0.0/4)

So you should never use these IP address spaces!

Public Versus Private IP Space

Misuse of Public IP address space can cause network routing problems for you network

Public IP Space

Prefix 10.0.0.0/8 has what subnet mask? The 8 says the first 8 bits must be ones So the first octet would be 255 and all

others would be zero 255.0.0.0

Subnet Mask

11111111128 64 32 16 8 4 2 1

What about 172.16.0.0/16? 192.168.0.0/24? 172.16.0.0/12?

Subnet Mask

Redefined how a traditionally Classful IP network could be used and subnetted (in equal size block)

With VLSM, subnets can be any size if they follow the binary rules

VLSM allows networks to be subdivided

Variable Length Subnet Mask (VLSM)

Old Classful Netorking

Variable Length Subnet Mask (VLSM)

Why is it Wrong

11000000 10101000 00000001 11000000

11111111 11111111 11111111 11111000

11111111 11111111 11111111 11110000

11000000 10101000 00000001 11001000

192.168.1.192

192.168.1.200

/29 255.255.255.248

/28 255.255.255.240

We use it all the time but do you really know what it is?

Classless Internet Domain Routing(CIDR)

CIDR is sort of the inverse of VLSM Where VLSM prescibes rules for subdividing

networks, CIDR prescribes rules for referencing groups of networks with a single route statement

Why would we want to do this?



Smaller routing tables are more logical, easier to understand, easier to troubleshoot, and require less CPU and memory for the routers.


what I just covered Check Second mistake I see is not understanding

public vs private vs reserved IP addresses Check

Third mistake I see is not understanding how to take a large prefix and break it down to usable network prefixes that allow for growth Not Yet

Fourth mistake I see is people not understanding why we do it


Allocate a block of IP addresses that can be referenced with a single access-list (filter) entry

Always allocate more IP addresses than requested

A Couple Rules First

Need 30 IP addresses for a server farm of database servers

Should we use a /27 255.255.255.224?


Allowing for 30 percent growth is a good rule of thumb

Round up to the next binary boundary 64 IP addresses or a /26 255.255.255.192


Now let say the server farm subnet was already allocated using 10.100.100.0/24

There are currently 10 servers in place .1 for the router and 2-11 for the servers You need to issue 30 more IP addresses on

this subnet Now what??? Just give them 12-42 right???

Allocate IP addresses that can be referenced with a single ACL

Allocate IP addresses that can be referenced with a single ACL

Allocate IP addresses that can be referenced with a single ACL Allocating groups of devices into

subnettable ranges◦ allows you to remove them from the network and

place them elsewhere without significant changes to the IP network design

You could allocate the range of 32-63◦ Access-list 101 permit ip any 10.100.100.32 255.255.255.224 eq

web So we are good right???

You should think ahead and allocate 64 IP addresses on a bit boundary

So you should allocate 64-127 Right???

◦ Access-list 101 permit ip any 10.100.100.64 255.255.255.192 eq web

Meets both rules so we are good???

What about rule 1?


what I just covered Check Second mistake I see is not understanding public

vs private vs reserved IP addresses Check Third mistake I see is not understanding how to

take a large prefix and break it down to usable network prefixes that allow for growth Not Yet

Fourth mistake I see is people not understanding why we do it Know why you are allocating the IP and allow for growth


There are three methods you can use to allocate IP addresses and IP subnets

Sequential– assign the first numerical subnet and then the next and so on, most commonly used. It is easy to understand

Divide by half - every time a network is allocated, the smallest available chunk is divided by half for use while preserving a large portion of IP address space for additional growth

Reverse binary – subnets are allocated by counting in binary with the most and least significant bits reversed. Is the most logical method, but is hard to understand

Have a Method to the Madness

Sequential IP Subnet Allocation

Some of us have been doing this for so long we remember the rule of all-zeros and all-ones as it relates to subnets

RFC 1878 states, “This practice (of excluding all-zeros and all-ones subnets) is

obsolete. Modern software will be able to utilize all definable networks.” Sometimes you maybe in an environment where

legacy equipment can not do this Or the staff still think they have to follow the

rule

Note about all-zeros and all-ones

Divide by Half

Reverse Binary

For the Cisco people in the class. You will need to know Cisco’s way to pass the CCNA

or know how to get the answer to the question based on how Cisco or vendor X tests

Everyone knows (or should now) that two IP addresses are used in every subnet (one for the gateway and one for broadcast)

Unless you have done enough networking to know you can use a /31 for to routers in a point-to-point connection. DO NOT ASKING IF YOU ARE NOT GOING TO TAKE CCDP!!!

IP Subnetting Made Easy

A /24 subnet has 256 host IP addresses – 254 IP are usable by host devices

Everything is based on the subnet masks which is based on binary

Everything will be powers of 2 and will either produce 256 or be divisible by 256

The maximum value of an octet is 255 (but remember we count from 0 so 256 number)


Subnet masks are, by their nature, inclusive There are only nine values that are possible

for any octet in a subnet mask


What the author is trying to say is a /24 or 255.255.255.0 would have 256 host with 16,777,216 possible subets (256*256*256*1)


I find an expanded for of the horizontal format very useful


IncrementNumber of hosts

128 64 32 16 8 4 2 1

CIDR /25 /26 /27 /28 /29 /30 /31 /32mask 128 192 224 240 248 252 254 255Usable Hosts

126 62 30 14 6 2 - -

csis 4823 data communications networking – ip addressing review

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