Mr. Mark Welton

 IPv4 address are 32-bit numbers represented in dotted decimal notation of 8 bit segments    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) Class A Class BClass C

 We are accustomed to the decimal system a base 10 system  The number is or ◦ 1x x x10 0 ◦ 1x100+2x10+4x1

 The binary number system is a base 2 system  is 0x2 7 +1x2 6 +1x2 5 +1x2 4 +1x2 3 +1x2 2 +0x x2 0  0x128+1x64+1x32+1x16+1x8+1x4+0x2+ 0x1  or

 So each octet (8-bit binary number) goes from ◦ –  So what is the value of

 =  So what is the hexadecimal value?  = 15 or F

 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

 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”  – (10/8 prefix)  – (172.16/12 prefix)  – ( /16)  These are the IP address spaces that can be used internally in an enterprise

 “link local” block ◦ – ( /16) ◦ To be used when a device can not get an IP address through DHCP  reserves lowest Class B ◦ ( /16) ◦ Not able to be used under old class system but can be assigned to someone now  Also defines loop back space (RFC 1700) ◦ – ( /8) ◦ Used for a machine to communicate internally  Also defines multicast address space (RFC 5771) ◦ – ( /4)  So you should never use these IP address spaces!

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

 Prefix /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 

 What about /16?  /24?  /12?

 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

/ /

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

 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.

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

 Need 30 IP addresses for a server farm of database servers  Should we use a / ?

 Need 30 IP addresses for a server farm of database servers  Should we use a / ?

 Allowing for 30 percent growth is a good rule of thumb  Round up to the next binary boundary  64 IP addresses or a /

 Now let say the server farm subnet was already allocated using /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 right???

 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 ◦ Access-list 101 permit ip any eq web  So we are good right???

 You should think ahead and allocate 64 IP addresses on a bit boundary  So you should allocate  Right??? ◦ Access-list 101 permit ip any eq web  Meets both rules so we are good???

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

 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

 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!!!

 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 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 Increment Number of hosts CIDR/25/26/27/28/29/30/31/32 mask Usable Hosts