CSIS 4823 Data Communications Networking – IP Addressing Review

1. CSIS 4823Data Communications Networking – IP Addressing Review Mr. Mark Welton

2. What is an IP address?

3. 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?

4. Somewhat Useless Trivia, Maybe? • 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 B Class C Class A 11000000 10101000 00000000 00000000

5. A Fast Binary Review • 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

6. A Fast Binary Review • The binary number system is a base 2 system • 011111002is 0x27+1x26+1x25+1x24+1x23+1x22+0x21+0x20 • 0x128+1x64+1x32+1x16+1x8+1x4+0x2+0x1 • 64+32+16+8+4 or 12410

7. A Fast Binary Review • So each octet (8-bit binary number) goes from • 000000002 – 1111111112 • So what is the value of 1111111112

8. A Fast Binary Review • 128+64+32+16+8+4+2+1 = 25510 • So what is the hexadecimal value? • 8+4+2+1 = 15 or F16 27 26 25 24 23 22 21 20 128 64 32 16 8 4 2 1 11111111

9. IP Address Allocation • 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

10. 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

11. Public Versus Private IP Space • 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

12. 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!

13. Public IP Space • Misuse of Public IP address space can cause network routing problems for you network

14. Subnet Mask • 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 • 255.0.0.0 128 64 32 16 8 4 2 1 11111111

15. Subnet Mask • What about 172.16.0.0/16? • 192.168.0.0/24? • 172.16.0.0/12?

16. Variable Length Subnet Mask (VLSM) • 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

17. Old ClassfulNetorking

18. Variable Length Subnet Mask (VLSM)

19. Variable Length Subnet Mask (VLSM)

20. Why is it Wrong 192.168.1.192 11000000 10101000 00000001 11000000 192.168.1.200 11000000 10101000 00000001 11001000 /29 255.255.255.248 11111111 11111111 11111111 11111000 /28 255.255.255.240 11111111 11111111 11111111 11110000

21. Classless Internet Domain Routing(CIDR) • We use it all the time but do you really know what it is?

22. 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?

23. Classless Internet Domain Routing(CIDR) Smaller routing tables are more logical, easier to understand, easier to troubleshoot, and require less CPU and memory for the routers.

24. IP Address Allocation • 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

25. A Couple Rules First • Allocate a block of IP addresses that can be referenced with a single access-list (filter) entry • Always allocate more IP addresses than requested

26. Always allocate more IP addresses than requested • Need 30 IP addresses for a server farm of database servers • Should we use a /27 255.255.255.224?

27. Always allocate more IP addresses than requested • Need 30 IP addresses for a server farm of database servers • Should we use a /27 255.255.255.224?

28. Always allocate more IP addresses than requested • 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

29. Allocate IP addresses that can be referenced with a single ACL • 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???

30. Allocate IP addresses that can be referenced with a single ACL

31. 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???

32. What about rule 1? • 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 eqweb • Meets both rules so we are good???

33. Maybe

34. IP Address Allocation • 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

35. Have a Method to the Madness • 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

36. Sequential IP Subnet Allocation

37. Note about all-zeros and all-ones • 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

38. Divide by Half

39. Reverse Binary

40. Reverse Binary

41. IP Subnetting Made Easy • 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!!!

42. 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)

43. IP Subnetting Made Easy • Subnet masks are, by their nature, inclusive • There are only nine values that are possible for any octet in a subnet mask

44. IP Subnetting Made Easy

45. IP Subnetting Made Easy • 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)

46. IP Subnetting Made Easy • I find an expanded for of the horizontal format very useful