Introduction to Industrial Ethernet, Part 5.

Part 4 was featured in Issue 6, MAYJUNE 2000. If you would like a copy, please send your request toinfo@ccontrols.com

INTRODUCTIONIn a previous article we discussedthe Internet Protocol and thestructure of IP addresses. An IPaddress identifies the source anddestination of a directed or unicastmessage and is defined in RFC 761.IPv4 is the most common versionof IP addressing requiring 32-bitaddresses. Although IPv6, the 128-bit version, will be used in thefuture, this article will restrict thediscussion to IPv4. IPv6 wasdeveloped because the explosivegrowth of the Internet will soondeplete the inventory of availableaddresses. At one time, 32-bitaddresses seemed to provide morethan enough addresses but therewas much waste in initialassignments and the class structureof IP addresses was inefficient. Inorder to make more efficient usageof IP address, the concept ofsubnetting was introduced withRFC 950. This article introducesthis concept.

Networks and Hosts

When we talk about a network weusually envision a cluster ofworkstations with one or moreservers connected to a local areanetwork. Each server andworkstation would have a unique

address to distinguish it from theother computers. With IPaddressing, servers andworkstations are all termed hostsbut each address not only identifiesa host but the address of thenetwork on which the host resides.This is because IP is aninternetworking protocol that notonly allows communicationbetween hosts on the samenetwork, but communicationbetween hosts on differentnetworks as well. The 32-bit IPaddress identifies a particular hostalong with the network on whichthe host resides. The structure of IPaddressing is defined so that anyhost on the public Internet can befound by any other host.

The format of the 32-bit address is and it is usuallyshown as four bytes of data.Although each byte could berepresented as a binary, decimal orhexadecimal number, the decimal-dot-decimal notation is the mostpopular. Therefore, the range of IPaddresses can span 0.0.0.0 to255.255.255.255. For example193.5.8.254 is a valid IP address butit is difficult to determine which partof the address is the network IDand which part is the host ID. Tounderstand the two you need toknow about class addressing.

Class Addressing

IPv4 is called a classful systemunder RFC 761 with IP addressesbeing defined as belonging to oneof five classes A, B, C, D or E.Classes A, B and C define differentpossible combinations of networkand host addresses. Class D isreserved for multicasting.Multicasting is the ability of onehost to communicate with manyother hosts with one transmissionand is beyond the scope of thisarticle. Class E is reserved for futureuse. The classes of interest tosubnetting are A, B and C.

With class A addresses, the first byteof the address identifies the networkaddress while the three remainingbytes identify the host. With class Baddresses, the first two bytesidentify the network address whilethe remaining two identify the hostaddress. With class C addresses, thefirst three bytes identify the networkaddress while the last byte identifiesthe host. That seems simple enoughbut how do you know you arelooking at either an A, B, C, D or Eaddress?

The four-byte IP address is viewedfrom left to right with the first byteon the left. This is the mostsignificant byte. The first few bits(most significant) of that byteidentify the class of address. For a

INTRODUCTION TO SUBNETTINGHow to maximize network addresses.By George Thomas,Contemporary Controls

Volume 1 Issue 8SeptemberOctober 2000

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theEXTENSIONA Technical Supplement to con t ro l NETWORK

2000 Contemporary Control Systems, Inc.

class A address, the left most bitmust be a zero. For a class Baddress, the first two bits must be a10. For a class C address, the firstthree bits must be a 110. For aclass D address, the first four bitsmust be a 1110. For a class Eaddress, the first four bits must be a1111. Therefore, it is only necessaryto observe the first byte of the IPaddress to determine its class.Figure 1 shows the decimal value ofthe first byte for each class.

Reserved Addresses

There are some reserved IP addressbesides those identified as classes Dand E. For example, the class Anetwork address 0.X.X.X cannot beused since it is used to indicatethis network. Class A address127.X.X.X is reserved for loop backtesting. With the host portion of theaddress, you cannot have an all 0shost, which refers to the networkaddress where the hosts reside.Likewise you cannot use the all 1shost address because that indicatesa broadcast which is a message toall hosts on the network. Therefore,with any host addressing on either aclass A, B or C network, you lose 2host addresses. Still with 4 billionpossible addresses from a 32-bitaddress space, you would thinkthere are plenty of addresses evenwith reserved addresses. Theproblem is that there was muchwaste when addresses wereoriginally assigned. For example, aclass A address can handle 16million hosts per one network ID.That is an enormous amount ofhosts for just one network. Even aclass B address can handle 65thousand hosts per network ID. Aclass C address can handle only 254hosts per network ID which may betoo little for some networks. Ascheme was needed to obtain abetter balance between network andhost assignments and that is calledsubnetting.

SUBNETTINGSubnetting creates additionalnetwork IDs at the expense of hostIDs and can be used with either A,B or C class addresses. If you lookat figure 2, you will notice that aclass B address uses 14 bits fornetwork addressing and 16 bits forhost addressing. By simplyreassigning one of the host bits to anetwork bit, you would double thenumber of available networkaddresses but halve the number ofhost addresses. Carrying theargument further, move eight of thehost bits (actually the complete thirdbyte) to the network side. The resultis 22 bits for network addressingand eight bits for host addressingwhich is quite similar to a class Caddress. These additional networkaddresses are called subnets and notnetworks because to the Internet,the original address is still a class Bnetwork address but locally theclass B network address can bebroken down to manageablesubnets that function as actualnetwork addresses. Why usesubnets? Subnets are interconnectedusing routers, and routers improvenetwork performance by reducingtraffic and minimizing disruptiondue to broadcast messages. Largenetworks become more manageablewhen subnets are deployed.

MASKINGTo create subnets you need asubnet mask that defines which bitswill be used to create the newnetwork address out of the 32-bit IPaddresses. By ANDing the 32-bitIP address with a 32-bit mask, wecreate a 32-IP address thatrepresents becoming our new network address.What do these masks look like? Ifwe start with a basic class A addressand do not define any subnets, themask would look like 255.0.0.0which is called a natural or default

mask. Only those bits that are set asa 1 will be considered whendefining a network address. In thiscase, all the bits in the first byte ofthe IP address will be considered.The natural mask for a class Baddress is 255.255.0.0 and for a classC address it is 255.255.255.0. Inorder to create more networkaddresses (subnets) we need tomove the mask bits to the right(changing 0 bits into 1s) in order toconvert host bits into network bits.The best way to understand theconcept is to use an example.

Assume we begin with IP address165.10.0.0. From figure 1 we knowthat this is a class B address with anetwork address of 165.10 with thecapability of assigning up to 65,534hosts. We do not want 65,534 hostson one network but would like tohave up to 500 hosts on eachsubnet. In order to have 500 hostson one subnet, we need to have 9bits of host addressing. Currently,we have 16 bits of host addressingsince we possess a class B address.That means that we can reassign 7of those bits to signify subnet bits.Therefore, the subnet mask wouldbe 255.255.254.0. In binary itwould be:

11111111.11111111.11111110.00000000

The natural mask for a class Baddress is 255.255.0.0 so in order tocreate subnets we moved mask bitsto the right in order to convert more

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Class A 1-126

Class B 128-191

Class C 192-223

Class D 224-239

Class E 240-254

Figure 1. The class of an IP addresscan be quickly identified byobserving only the first byte.

host bits to network bits. It must beremembered that these mask bitsmust be contiguous from the left.For example, the above mask allowsup to 510 host assignments.Remember that we cannot useeither an all 0s or all 1s hostaddress. The next jump would beto allow up to 1022 host addresses.What would be the subnet mask? Itwould be 255.255.252.0. The 1s arestill contiguous from the left. Thisapproach creates many subnets, butit is recommended that neither anall 0s nor all 1s subnet be used.This could cause a problem onsome networks. How many maskbits can you have? You need tohave some hosts on a network andtwo host addresses are unusable sothe maximum number of mask bitsis 30 leaving two valid hostaddresses.

NOTATIONUsing the last subnet mask in theabove example, we have 1022 hostaddresses. What if our computeractually had host address 768 onsubnet 4? What would be our actualIP address? We cannot say it is165.10.4.768 since with decimalnotation no byte can be more than255. The actual IP address would be165.10.7.0 so you do need to knowthe subnet mask before determiningthe actual subnet address and hostaddress.

There is a simpler way ofrepresenting the actual IP addressand that is by using the ClasslessInterDomain Routing (CIDR)scheme. With this scheme theconcept of A, B and C classes iseliminated, but the concept ofsubnetting is retained. In the aboveexample, we use a total of 22 bits ofcontiguous 1s in our mask so wewould display our IP address as165.10.7.0/22. Although it is still notobvious that we are host 768 onsubnet 4 of network 165.10, we can

figure it out using this singlenotation which tells us exactlywhere the subnet mask separatesthe network and host addresses.

For example, in a previous articlewe mentioned that there were oneA, 32 B and 256 C addresses thatwere strictly private and cannot beaccessed through the Internet. Theseare as follows:

10.0.0.0 to 10.255.255.255

172.16.0.0 to

172.31.255.255

192.168.0.0 to 192.168.255.255

Notice that the first range is asingle A address with 24 bits ofhost addressing, the second are Baddresses with 16 bits of hostaddressing and the third are Caddresses with 8 bits of hostaddressing. Using CIDR notationthese same address ranges can bedisplayed as follows:

10.0.0.0/8

172.16.0.0/12

192.168.0.0/16

The natural mark for a class Aaddress is 255.0.0.0 which meanseight contiguous 1s from the left so10.0.0.0/8 represents the naturalmask for a class A address. This iswhat we would expect. A singleclass A network address withprovisions for 24 bits of host

addressing. The natural mask for aclass B address is 255.255.0.0 which,with CIDR notation, would be /16but the above class B addresseshave only 12 mask bits ofcontiguous 1s. This seems to violateour rule for subnetting and it does.With subnetting you move the bitsto the right of the natural maskthereby consuming host bits. Insteadwe are moving the mask to the leftof the natural mask (changing 1 bitsto 0s) consuming network bits. Thisis called supernetting which requirescontiguous network addresses andwill be discussed shortly. By movingthe mask to the left by four bitsfrom the natural mask, we can gainmore host addresses at the expenseof 16 contiguous network addresses.Therefore, the notation 172.16.0.0/12is short for indicating a range ofcontiguous network addresses from172.16.0.0 to 172.31.0.0. The same istrue for the last example which areC class addresses. The natural maskfor a C address is /24. Instead theCIDR notation is a /16 meaningeight less mask bits thereby yieldinga range of network addresses from192.168.0.0 to 192.168.255.0.

SUPERNETTINGThe inverse of subnetting issupernetting. Instead of movingmask bits to the right of the naturalmask for subnetting, we move maskbits to the left for supernetting. Withsubnetting we create more network

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Address Network Address Host AddressIdentifier

Class A 0 7 bits of network address 24 bits of host addressFirst byte Last three bytes

Class B 10 14 bits of network address 16 bits of host addressFirst two bytes Last two bytes

Class C 110 21 bits of network address 8 bits of host addressFirst three bytes Last byte

Class D 1110 Multicast address in the range of 224.0.0.0 239.255.255.255

Class E 1111 Class E Reserved for future use

Figure 2. Address classes define the split between network and host IDs.

addresses at the expense of hostaddresses. With supernetting wecreate more host addresses at theexpense of network addresses.Supernetting is not for users since itwould be difficult for users to begranted a range of contiguousnetwork addresses. Supernetting isfor Internet Service Providers (ISPs)who are attempting to obtain themost efficient allocation of IPaddresses using the A, B, C classscheme.

SUMMARYAlthough a 32-bit IP address offersan extremely large number ofaddresses, the A, B, C, classstructure does not make efficientuse of assignments.Subnetting improves the situation byallowing a finer split betweennetwork and host assignments whileimproving the performance andmaintainability of large networks.

REFERENCESIllustrated TCP/IP, Matthew Naugle, 1998, Wiley Computer Publishing

Practical Networking With Ethernet, Charles E. Spurgeon, 1997,International Thomson Computer Press

International Standard ISO/IEC 8802-3 ANSI/IEEE std 802.3, 1996, The Institute of Electrical and Electronic Engineers, Inc.

TCP/IP Clearly Explained, Pete Loshin, 1997, Academic Press

TCP/IP Illustrated, Volume 1, The Protocols, W. Richard Stevens, 1994,Addison-Wesley Publishing Company

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Subnet mask CIDR # Subnets # Host

11111111.11111111.11111111.00000000 /24 0 254

11111111.11111111.11111111.11000000 /26 2 62

11111111.11111111.11111111.11100000 /27 6 30

11111111.11111111.11111111.11110000 /28 14 14

11111111.11111111.11111111.11111000 /29 30 6

11111111.11111111.11111111.11111100 /30 62 2

The natural mask for a class C address is 255.255.255.000 which providesfor up to 254 host addresses. By moving the mask bits to the right(replacing 0s for 1s), subnets are created at the expense of host bits. Notshown are masks /25 and /31 since they are not allowed. Similar chartscan be made for class A and class B addressing. Class A subnetting beginsat /10 and class B at /18. Both end at /30.

SUBNETTING A CLASS C ADDRESS

CoverINTRODUCTIONNetworks and HostsClass AddressingReserved Addresses

SUBNETTINGMASKINGFig 1 - IP Address Classes

NOTATIONFig 2 - Classes Split Network and Host IDs

SUPERNETTINGSUMMARYSUBNETTING A CLASS C ADDRESS

REFERENCES