2 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 IP Addressing

333 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 IP Addressing Basic Addressing Working with Addresses Summarization & Subnets VLSM Working with VLSM Networks Classful Addressing Working with Classful Addressing

444 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 Basic Addressing IP addresses are written in dotted decimal format. Four sections are separated by dots. Each section contains a number between 0 and 255. Dots separate the sections Each section contains a number between 0 and 255

555 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 Basic Addressing Why is each section a number between 0 and 255? Computers operate in binary, humans operate in decimal. Computers treat IP addresses as a single large 32 digit binary number, but this is hard for people to do. So, we split them up into four smaller sections so we can remember and work with them better! Dots separate the sections Each section contains a number between 0 and 255 Why????

666 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 Basic Addressing /4 == = 256. But, computers number starting at 0, so to make a space of 256 numbers, we number from 0 to Each 8 digit group represents a number between 0 and 255

777 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 Basic Addressing Each device on a network is assigned an IP address. Each IP address has two fundamental parts: The network portion, which describes the physical wire the device is attached to. The host portion, which identifies the host on that wire. How can we tell the difference between the two sections? NetworkHost

888 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 Basic Addressing The network mask shows us where to split the network and host sections. Each place there is a 1 in the network mask, that binary digit belongs to the network portion of the address. Each place there is a 0 in the network mask, that binary digit belongs to the host portion of the address NetworkHost

999 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 Basic Addressing An alternative set of terminology is: The network portion of the address is called the prefix. The host portion of the address is called the host. The network mask is expressed as a prefix length, which is a count of the number of 1’s in the subnet mask PrefixHost = /24

10 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 Basic Addressing The network address is the IP address with all 0’s in the host bits. The broadcast address is the IP address with all 1’s in the host bits. Packets sent to either address will be delivered to all the hosts connected to the wire / prefixhost these bits are 0, so this is the network address / prefixhost these bits are 1, so this is the broadcast address

11 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 Working with Addresses Two of the most common questions you are going to face when dealing with IP addresses are: What’s the network? What’s the host? How dow we figure this out? /26 ????

12 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 Working with Addresses (The Hard Way) First, convert the IP address into binary. This is easier than it looks. Work with one octet at a time. Divide by two, farm out the remainder on the side. The bottom is the binary MSD, the top the binary LSD divide by 2 remainder 48 0 divide by 2 remainder 240 divide by 2 remainder 120 divide by 2 remainder 60 divide by 2 remainder 30 divide by 2 remainder 11 divide by 2 remainder 01 divide by 2 remainder Left Right

13 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 Working with Addresses (The Hard Way) Write down the IP address If you have a prefix length, just wrote down the number of 1’s. If you have a network mask, computer the binary as with the IP address == 26 AND these two Convert back to dotted decimal. This is the network address

14 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 Working with Addresses (The Hard Way) Write down the IP address If you have a prefix length, just wrote down the number of 1’s. If you have a network mask, computer the binary as with the IP address == 26 NOR these two Convert back to dotted decimal. This is the host address

15 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 Working with Addresses (The Hard Way) To convert from binary to decimal, use a simple chart. Add the number indicated for each 1 set in the binary number

16 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 Working with Addresses (The Easy Way) First, if you are using a network mask, convert it to a prefix length. For each octet in the network mask that is 255, add 8 to the prefix length. For the one octet that isn’t 255, convert to binary and add the right number of bits--or use a chart! == ==

17 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 Working with Addresses (The Easy Way) Take the prefix length and divide by 8. Take the resulting number, and ignore those octets out of the IP address-- these are all part of the network address! We’re going to use the remainder to find the fourth octet of the network address. 26/8 == 3 (remainder 2) /26 These three octets are part of the network The remainder tells us what the network address in the fourth octet is

18 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 Working with Addresses (The Easy Way) Take the remainder, and find the corresponding “multiple” on the chart; in this case, 64. The largest multiple of 64 that will fit into 80 is 64, so the network is 64. Add the three octets we “set aside” earlier, and the network (prefix!) is / == 16, so the host address is x 1 == x 2 == 128 Remainder == 2 Network is 64! == /26 16 Hosts!

19 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 Working with Addresses (The Easy Way) How many hosts are in this network? The remainder tells us there are 64 addresses, minus the network and broadcast addresses, so 62 hosts. To find the broadcast address, subtract 1 from the number of hosts, and add that number to the network address. The key is to work in octets, rather than trying to work with the entire IP address at once! Remainder == == 62 hosts 64 addresses 64 + (64 - 1) == is the broadcast address

20 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 Working with Addresses (The Easy Way) What if the prefix length is less than 24? Take the prefix length and divide by 8. Take the resulting number, and ignore those octets out of the IP address-- these are all part of the network address! We’re going to use the remainder to find the third octet of the network address. 22/8 == 2 (remainder 6) /22 These three octets are part of the network The remainder tells us what the network address in the third octet is

21 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 Working with Addresses (The Easy Way) Take the remainder, and find the corresponding “multiple” on the chart; in this case, 4. The largest multiple of 64 that will fit into 80 is 64, so the network is 64. Add the two octets we “set aside” earlier, and make any octets after the network 0’s (the fourth octet). The network (prefix!) is / x 25 == x 26 == 104 Remainder == 6 Third octet is 100! Set the fourth octet to /22

22 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 Working with Addresses (The Easy Way) To find the number of hosts, take the number of octets set to 0, which is 1 in this case (the fourth octet), and multiply by 256. Next, take the number relating to the remainder from the chart, and multiple this by the number we just found above. Subtract two x 256 == – 2 == 1022 hosts Remainder == 6 “0” octets == 1 1 x 256 == 256

23 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 Working with Addresses (The Easy Way) The key is to work in octets, rather than trying to work with the entire IP address at once!

24 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 Summarization & Subnets A single network address (prefix!) represents a set of hosts attached to a wire. We can abstract this, and simply say that a prefix represents a set of reachable addresses. We can say that we’ve “summarized” information about the hosts attached to the physical wire by referring to the entire group as a single network /26

25 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 Summarization & Subnets In effect, we’ve shortened the network part of the address (prefix!), and lengthened the host portion of the address, in effect describing more hosts (destinations) in a single address. If we can shorten the prefix length to describe multiple hosts with a single network address, why can’t we shorten the prefix length so a single network address describes two networks? We can! It’s called address summarization, or just summarization / / / / / /32 These host addresses are described by this network /25 These networks are described by this network

26 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 Summarization & Subnets through through through , so it’s the same space! Changing the mask bit from 1 to 0, which shortens the prefix length, means the bit in the two networks that distinguish them from one another are now considered host bits!

27 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 Summarization & Subnets A network which is a part of another network is called a subnet. There is another term, the supernet, but it’s definition depends on whether you are using VLSM subnetting, or calssful subnetting, so it will be defined in the next two sections / / / / / /32 These host addresses are subnets of this network /25 These networks are subnets of this network

28 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 VLSM VLSM: Variable Length Subnet Masking It simply means that the entire IP address space is treated as one flat address space. Any prefix length is allowed in the network at any point / / / /27 All of these are valid in the same network!

29 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 VLSM At this point, you pretty much already know VLSM! You already know how to find the network address, broadcast address, and number of hosts in a network. Two other common problems in working with VLSM networks remain: Building summary addresses from groups of networks. We won’t cover this here (maybe later in routing). Building network addressing schemes from a given number of hosts and networks.

30 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 Working with VLSM Networks You have 5 subnets with the following numbers of hosts on them: 58, 14, 29, 49, 3 You are given the address space /24. Determine what subnets you could use to fit these hosts into it. How to solve this: Start with the chart! Order the networks from the largest to the smallest. Find the smallest number in the chart that will fit the number of the largest number of hosts + 2. Continue through each space needed until you either run out of space, or you finish.

31 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 Working with VLSM Networks 58, 14, 29, 49, 3: reorder to 58, 49, 29, 14, 3. Start with 58. Smallest number larger than (58 + 2) is is 2 bits. 24 bits of prefix length in the address space given, add 2 for 26. First network is /26. The next network is , so we start the next “round” at < (58 + 2) < == /26 takes care of the first 58 hosts Start the next block at

32 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 Working with VLSM Networks Next block is 49 hosts. Smallest number larger than (49 + 2) is is 2 bits. 24 bits of prefix length in the address space given, add 2 for 26. We start this block at , so network is /26. The next network is , so we start the next “round” at < (49 + 2) < == /26 takes care of the next 49 hosts Start the next block at

33 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 Working with VLSM Networks Next block is 29 hosts. Smallest number larger than (29 + 2) is is 3 bits. 24 bits of prefix length in the address space given, add 3 for 27. We start this block at , so network is /27. The next network is , so we start the next “round” at < (29 + 2) < == /27 takes care of the next 29 hosts Start the next block at

34 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 Working with VLSM Networks Next block is 14 hosts. Smallest number larger than (14 + 2) is is 4 bits (actually equal, but it still works!). 24 bits of prefix length in the address space given, add 4 for 28. We start this block at , so network is /27. The next network is , so we start the next “round” at (14 + 2) == == /28 takes care of the next 14 hosts Start the next block at

35 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 Working with VLSM Networks Last block is 3 hosts. Smallest number larger than (3 + 2) is 8. 8 is 5 bits. 24 bits of prefix length in the address space given, add 5 for 29. We start this block at , so network is /29. This is the last block of hosts, so we’re done! < (5 + 2) < == /29 takes care of the next 14 hosts

36 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 Working with VLSM Networks A subnet is any network which is “part of” a larger network space. A supernet is any network which covers a larger space than a given network, including the space covered by the network / / / / / /26 subnets subnet supernet

37 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 Classful Addressing Classful subnetting is similar to VLSM, with two more rules: The IP address space is divided into “classes,” with each class having a specific “natural” prefix length. Each block of address space is called a “major net.” You cannot have more than one prefix length within a major net.

38 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 Classful Addressing Network ClassBeginning Digits in Binary Natural Prefix Length Range of Addresses Example Major Networks Class A10XX /8 through / / / /8 Class B110X /16 through / / / /16 Class C /24 through / / / /24

39 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 Classful Addressing It’s illegal to have multiple network masks within a single major network. There cannot be a mix of /24’s and /25’s in the /8 major network. There cannot be a mix of /25’s and /26’s in the /8 network / / / / / /26 two different prefix lengths in the same major network

40 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 Working with Classful Addressing You can find the network address, broadcast address, and number of hosts as we described earlier. You can find the number of networks by subtracting the network mask from the natural mask, and then using the chart.

41 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 Working with Classful Addressing /25 is in the class A major network. The natural prefix length for a class A network is /8. Subtract the natural prefix length from the actual prefix length. Divide by 8, holding the remainder on the side / /8 is class A 25 – 8 == 17 17/8 == 2, 1 remaining

42 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 Working with Classful Addressing Find the remainder in the power of two’s chart. Multiply the result, 256, and the number from the power of two’s chart. Subtract (256 x 2) x 128 == / /8 is class A 25 – 8 == 17 17/8 == 2, 1 remaining – 2 == networks

43 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 Working with Classful Addressing Subnet 0 The network with all the between the host and the natural major net set to 0. This only exists in classful addressing schemes / natural network natural host configured network these bits are 0, so this is subnet / / / / /25 Yes No Yes No Yes

44 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 Working with Classful Addressing Broadcast Subnet The network with all the bits between the host and the natural major network set to 1. This only exists in calssful address schemes / natural network natural host configured network these bits are 1, so this is the broadcast network / / / / /25 Yes No Yes No Yes

45 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 Working with Classful Addressing You have 5 subnets with the following numbers of hosts on them: 58, 14, 29, 49, 3 You are given the address space /22. Determine what subnets you could use to fit these hosts into it. How to solve this: Start with the chart! Find the largest set of hosts. Find the smallest number in the chart that will fit the number of the largest number of hosts + 2. Use that prefix length for all the subnets (remember you cannot have different subnet masks within the same major network).

46 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 Working with Classful Addressing A subnet is any prefix with a prefix length longer than the natural prefix length of the major network. A supernet is any prefix with a prefix length shorter than the natural prefix length of the major network /24Subnet /9Subnet /15Supernet /25Subnet /23Supernet

47 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 Private & Special Address Space Address SpaceRange of Addresses / through / through / through Network ClassBeginning Digits in BinaryRange of Addresses Class D (Multicast) 11110x through Class E (Experimental) 11111x through....

48 © 2003, Cisco Systems, Inc. All rights reserved. RST-2002 Cisco IOS Show IP Route 2651A#sho ip route.... Gateway of last resort is not set C /24 is directly connected, Serial0/2.... S /24 [1/0] via /16 is variably subnetted, 2 subnets, 2 masks S /24 [1/0] via S /29 [1/0] via C /24 is directly connected, Serial0/0 C /24 is directly connected, FastEthernet0/0 C /24 is directly connected, FastEthernet0/1 S /16 [1/0] via two different prefix lengths under the same major network a supernet and natural mask in the same network address space