TCP/IP Subnetting

ITC542-201460 Assessment II

Rashif Rahman, Charles Sturt University

Student ID: 11556946, Due: September 8, 2014

1 Switching

1.1 Packet Switching

The concept of packet switching began with the ideas of independent researchers (Kurose& Ross, 2012, p. 60). When computers were becoming important for science in the middleof the 20th century, it also became apparent that there was a need to interconnect systemsregardless of distance. The then-existing communication technique of circuit switchingused for telephony proved to be incompatible with the very nature of computing (Leineret al., 2009; Roberts, 1978). Ideas were put into practice in the 1970s with the inventionof what is now known as the TCP/IP protocol suite (Forouzan, 2009, p. 3). The newswitching technique uses packets rather than circuits, in that messages are dividedinto small pieces of data and moved across a physical medium (Forouzan, 2009, p. 97;Tanenbaum & Wetherall, 2010, pp. 162164). Being based on digital technology meantthat the switching could be independent of the physical medium.The purpose of this new switching technique was twofold: networking and internetworkingor, allowing devices to talk to each other, and allowing networks of devices to talk to eachother (a network of networks), efficiently (Leiner et al., 2009; Roberts, 1978). The keyhere is efficiency. Traditional circuit switching reserves the path when a connection isestablished, so no two connections can use the same path. In packet switching paths canbe used by multiple connections. Any link in the path can contain packets belonging todifferent connections. In this way, packets of one connection are all independent and maytake different routes to a destination. This is a best-effort delivery, and is compatible withthe bursty networking pattern of computers, where any link can be idle for a long timewith sudden bursts of traffic occasionally. In this idle time, other connections can use thelink.

1.2 Circuit Switching

Switching allows interconnecting multiple devices without having to set up expensivepoint-to-point infrastructure (Forouzan, 2006, 8). Circuit switching was the naturalevolution of telecommunications technology from the telegraph (Holzmann & Pehrson,1994), necessitated by the proliferation of telephony and made possible by 20th centuryengineering (AT&T, n.d.; COMNEWS, 1984). Although the technical details can be

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complex, the conceptlike many scientific breakthroughsis simple in hindsight. Thereare three phases to a circuit-switched connection: a physical electrical circuit is establishedbetween two ends, communication ensues, and the circuit is freed once the communicationis over (Forouzan, 2006, 8.1; Kurose & Ross, 2012, 1.3.2; Tanenbaum & Wetherall, 2010,2.6.5). Switches in the path between the two end points determine which links must beconnected in order to close the circuit (Forouzan, 2006, p. 217, 2009, p. 132).A circuit-switched connection is therefore temporary, but dedicated. A different pair ofend points will use a different set of links, but each connection is unique and uses thesame set of links every time. The physical path comprising a set of links and switches istherefore predetermined for every unique source and destination. At any point in timethe overall circuit is either closed or open; a path is either used or lies dormant. Becausethere is no sharing of links for different paths, throughput is guaranteed at all times.Communication can occur in real-time since resources are reserved during connectionestablishment. Traditional public telephone networks (plain old telephone service, orPOTS) are a prime example of circuit switching, although computer networks can alsobe circuit-switched, and modern telephone networks today are moving away from thistechnique (Forouzan, 2006, 9; Tanenbaum & Wetherall, 2010, p. 68, 164).

1.3 Packets vs. Circuits

Digital information can represent different types of data and activity that can have differentrequirements of the underlying communication mechanism. Because packet switchingabstracts away the physical medium, it is an appropriate switching technique for digitaldata communications. Even when the physical layer of the communication is circuit-switched, packetized data enables greater link utilization. For example, a circuit-switchedlink can be divided up into channels and analogue signals can use one channel whiledigital (packetized) data can use another (Forouzan, 2006, 6, 9). Because much digitalactivity is bursty in nature, a link is not always busy. Breaking up data into smallerpackets enables a link to accommodate many such packets from different connections. Thismaximizes utilization by minimizing idle time. Individual packets follow an optimizedroute, taking alternative routes when needed. This provides a layer of redundancy.Circuit switching may be appropriate for analogue communication where there is onlyone class of signal and the network is dependent upon a physical layer of signal routinglogic, or for communication where a direct end-to-end link is required for reasons such asprivacy and quality of service. However, because a circuit-switched link is reserved forone connection, it is not suitable for communication that is bursty in nature and wherepermanent allocation of resources would be wasteful or expensive. It is also not suitablefor short messages due to the overhead of setting up a connection. Circuit switching isinherently not fault-tolerant, as any problem along the path of a connection will preventpassage of information and require costly physical repairs. There is no redundancy; if onelink is damaged, there is no alternative. With the advances in technology today, evenprivacy and quality of service can be met by packet switching (Ars Technica, 2013).

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2 Subnetting

Available Block: 110.50.0.0/16

No. of Addresses: 65,536 (23216)Network Mask: 255.255.0.0 (16-bit prefix)Address Range: 110.50.0.0 110.50.255.255

Prospective Customers: 2,600

Group 1: 200 businesses 120 addressesAllocatable Addresses: 200 128 (2dlog2 120e) = 25, 600Addresses Unused: 25, 600 (200 120) = 1, 600

Group 2: 400 businesses 20 addressesAllocatable Addresses: 400 32 (2dlog2 20e) = 12, 800Addresses Unused: 12, 800 (400 20) = 4, 800

Group 3: 2000 households 6 addressesAllocatable Addresses: 2000 8 (2dlog2 6e) = 16, 000Addresses Unused: 16, 000 (2000 6) = 4, 000

The ISP has a /16 CIDR block granted to it, so it has a total of 65,536 addresses (prefixlength subtracted from bit length of IPv4 address space, as a power raised from two)to use for its business. Blocks allocated to each customer must be a power of two.Mathematically, this is because the number of addresses N needs to provide an integervalue for prefix length n; since N = 232n, solving for n yields n = 32 log2N . Therefore,the requested number of blocks are transformed to comply with this requirement. Thenext power of two of any number i can be found by calculating the ceiling of the base-2logarithm of i to get an integer j, and then raising two to the power of j.

Table 1: IP address allocation across three groups

Group Businesses Users Subnet Unused1 200 120 110.50.0.0/25 16002 400 20 110.50.100.0/27 48003 2000 6 110.50.150.0/29 4000

2.1 Group 1

Subnet Address: 110.50.0.0/25

Subnet Range: 110.50.0.0 110.50.99.255

Block Size: 128

Prefix Length: 25 (32 log2 128)

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Network Mask: 255.255.255.128

11111111.11111111.11111111.10000000 (25-bit prefix)255. 255. 255. 128 (decimal value)

This group has 200 customers (medium-size businesses) and each customer needs 120addresses. However, the number of addresses allocated (block size) per customer is 128.This is because 120 is not a power of two, and 128 is the next power of two after thatnumber (2dlog2 120e). Therefore, the total number of addresses allocated for Group 1 is25,600 (200 128). The prefix length is found by subtracting the base-2 logarithm ofthe block size from the bit length of the IPv4 address space. The network mask is thebase-256 decimal equivalent of an address with 25 bits all set to TRUE (1) and the rest(32 25 = 7) to FALSE (0). Twenty-four bits would cover up to the third byte (octet),leaving one bit for the next (last) octet. Since this bit is in the 27 place and no otherbit in the byte is set, the octets value is just 128. The network mask can be used todetermine the last address of a block.

2.1.1 First Customer

Allocated Block: 110.50.0.0/25

First Address: 110.50.0.0 (network address)

Last Address: 110.50.0.127 (special address)

00000000 (last octet of first address)OR

01111111 (NOT of last octet of mask)--------01111111 = 127

The starting address for the first customer is simply the network address of the parentblock (since it is the first allocation), except with a larger prefix. Since 25 bits are TRUE(dictated by the mask), the first three octets of the address keep their values. The fourthoctet is not affected because its value is zero. The last address can be found by simplyadding block size - 1 to this address. More precisely, it is also found by applying abinary OR operation over the network (first) address and binary NOT (complement) ofthe network mask. The TRUE octets will always yield the same values in this operation,so a short-cut is to only apply it to the octet that is partially TRUE (less than eight bitsTRUE). In this operation, any remaining right-most octets will simply be TRUE.

2.1.2 nth Customer

Allocated Block: 110.50.x.y/25110.50.0.0256 + base-256(n 1 block size), e.g. when n = 200, first solve for theright-hand expression:

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(200 1) 128 = 2547225472 mod 256 = 128b25472 256c = 99

Then, carry out the base-256 addition:

110. 50. 0. 0+ 0. 0. 99.128---------------110. 50. 99.128

:: x = 99, y = 128





The address block of the nth customer can be found by simply multiplying the blocksize by n 1. The result will be a base-10 decimal number, which can be converted tobase-256 and added to the first address of the group to yield the starting address of theblock. All other operations remain the same as with the first block. Only the third andfourth octets change because 16 bits are available for the ISP to use across all groups ofcustomers. Here the results are for the last customer in this group, so the next allocationcan continue from the next available block 110.50.100.0.

2.2 Group 2


Subnet Range: 110.50.100.0 110.50.149.255

Block Size: 32

Prefix Length: 27 (32 log2 32)Network Mask: 255.255.255.224


The total number of addresses allocated for Group 2 is 12,800 (400 32). The networkmask is the base-256 decimal equivalent of an address with 27 bits all set to TRUE (1)and the rest (32 27 = 5) to FALSE (0). Twenty-four bits would cover up to the thirdbyte (octet), leaving three bits for the next (last) octet. Since these bits are in the 27, 26and 25 place, the octets value is 224.

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The starting address for the first customer is simply the subnet address of the group (sinceit is the first allocation in this group). Since 27 bits are TRUE, the first three octets of theaddress keep their values. The fourth octet is not affected because its value is zero.

2.2.2 nth Customer

Allocated Block: 110.50.x.y/27110.50.100.0256 + base-256(n 1 block size), e.g. when n = 400, first solve for theright-hand expression:

(400 1) 32 = 1276812768 mod 256 = 224b12768 256c = 49


110. 50.100. 0+ 0. 0. 49.224---------------110. 50.149.224

:: x = 149, y = 224





Here the results are for the last customer in this group, so the next allocation can continuefrom the next available block 110.50.150.0.

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2.3 Group 3


Subnet Range: 110.50.100.0 110.50.

Block Size: 8

Prefix Length: 29 (32 log2 32)Network Mask: 255.255.255.248


The total number of addresses allocated for Group 3 is 16,000 (2000 8). The networkmask is the base-256 decimal equivalent of an address with 29 bits all set to TRUE (1)and the rest (32 29 = 3) to FALSE (0). Twenty-four bits would cover up to the thirdbyte (octet), leaving five bits for the next (last) octet. Since the 20, 21 and $22 placesare zero, the octets value is 248.







The starting address for the first customer is simply the subnet address of the group.Since 29 bits are TRUE the first three octets of the address keep their values. The fourthoctet is not affected because its value is zero.

2.3.2 nth Customer

Allocated Block: 110.50.x.y/29110.50.150.0256 + base-256(n 1 block size), e.g. when n = 2000, first solve forthe right-hand expression:

(2000 1) 8 = 1599215992 mod 256 = 120b15992 256c = 62


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110. 50.150. 0+ 0. 0. 62.120---------------110. 50.212.120

:: x = 212, y = 120





The results are for the last customer in this last group, so any new allocations can continuefrom the next available block 110.50.212.128.

3 Forwarding

3.1 Router R1

The router first determines the class of the destination address so that it can extractthe network address and search the appropriate routing table for where to forward thepacket next. This is done by bit-shifting the address 28 bits to the right. The address145.80.23.14 is 10010001 01010000 00010111 00001110 in binary. The result of thebit-shift is 00000000 00000000 00000000 00001001, or 9, which indicates a Class Bnetwork. The router then extracts the network address by performing a binary ANDoperation over the Class B network mask and the destination address.

10010001 01010000 00010111 00001110 (145.80.23.14)11111111 11111111 00000000 00000000 (255.255.0.0)================>------------------10010001 01010000 00000000 00000000 (145.80.0.0)

The result of the masking is 145.80.0.0. The router searches the Class B routing tableand finds it in the first row. The next-hop address is empty, which means the destinationnetwork is connected to the router and a direct delivery can be made. The destinationaddress and interface number m1 are passed to ARP to retrieve the physical address ofthe next hop.

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Table 2: Class A table of router R1

NetworkAddress

Next-hopAddress Interface

111.0.0.0 m0

Table 3: Class B table of router R1

NetworkAddress


145.80.0.0 m1170.14.0.0 m2

Table 4: Class C table of router R1

NetworkAddress


192.16.7.0 111.15.17.32 m0

Default: 111.30.31.18, m0

3.2 Router R2

The router determines the class of the destination address by bit-shifting the address 28bits to the right. The address 145.80.23.14 is 10101010 00001110 00011000 00001100in binary. The result of the bit-shift is 00000000 00000000 00000000 00001010, or 10,which indicates a Class B network. The router then extracts the network address byperforming a binary AND operation over the Class B network mask and the destinationaddress:

10101010 00001110 00011000 00001100 (170.14.24.12)11111111 11111111 00000000 00000000 (255.255.0.0)================>------------------10010001 01010000 00000000 00000000 (170.14.0.0)

The result of the masking is 170.14.0.0, and the router searches the Class B routing table,finding the address in the second row. There is next-hop address, which means an indirectdelivery must be made. The address 111.25.19.20 and interface number m1 are passed toARP to retrieve the physical address of the next hop.

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NetworkAddress


111.0.0.0 m1


NetworkAddress


145.80.0.0 111.25.19.20 m1170.14.0.0 111.25.19.20 m1


NetworkAddress


192.16.7.0 111.15.17.32 m1

Default: 0.0.0.0, m0

3.3 Router R3

The router determines the class of the destination address by bit-shifting the address 28bits to the right. The address 135.15.42.18 is 10000111 00001111 00101010 00010010in binary. The result of the bit-shift is 00000000 00000000 00000000 00001000, or 8,which indicates a Class B network. The router then extracts the network address byperforming a binary AND operation over the Class B network mask and the destinationaddress:

10000111 00001111 00101010 00010010 (135.15.42.18)11111111 11111111 00000000 00000000 (255.255.0.0)================>------------------10010001 01010000 00000000 00000000 (135.15.0.0)

The result of the masking is 135.15.0.0, and the router searches the Class B routing table,but does not find the address. This means that the network is somewhere else in theInternet and the packet must be forwarded to the default router. The address 111.30.31.28and interface number m1 are passed to ARP to retrieve the physical address of the nexthop.

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NetworkAddress


111.0.0.0 m1


NetworkAddress


145.80.0.0 111.25.19.2 0 m1170.14.0.0 111.25.19.2 0 m1


NetworkAddress


192.16.7.0 m0

Default: 111.30.31.18, m1

4 ARP

The ARP protocol is simply a helper protocol of the network layer, sitting closer to thedata link layer (Forouzan, 2009, p. 222). Its job is to facilitate node-to-node passingof data by retrieving the physical (hardware) address that relates to a logical address.(Forouzan, 2009, 6.2) The word relates is used because the physical address retrievedmay not necessarily be that of the device with that particular logical addressit can beanother device that acts on behalf as a forwarder (e.g. a router among many routers ina path towards the device). Every time the network layer prepares data to be sent out,ARP helps it fill in the hardware address that the data link layer will use (Forouzan, 2009,pp. 4850).It does this by creating a query message of its own that will be broadcast (Figure 1). Thecorrect system (it can be a host or a router acting on behalf of the host) will respond withits logical address mapped to a hardware address. In effect, ARP basically asks everyonearound whether one of them is or has access to a particular logical address. In the reply(Figure 2), the broadcast address field is replaced with the original senders address (i),while the addresses in the payload change places (j) and the the new physical address isinserted into the source address (k). The requester now knows that this address is to bemapped to the logical address it did not have information about.

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Figure 1: ARP Request Encapsulation

Figure 2: ARP Reply Encapsulation

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5 ICMP

5.1 The Protocol

ICMP messages can be categorized as either error-reporting messages or query messages.Several message types are defined, and within each type there are codes indicatingthe reason for the message (RFC792, 1981). Error-reporting messages contain usefulinformation while query messages can glean useful information. An error-reporting messageincludes part of the original datagram for identification purposes and can inform thesender when a destination is unreachable, when a router has redirected a datagram, orwhen a datagram header contains invalid data. It can report when a datagram is stuck ina loop or when fragments do not arrive on time. It can also warn the sender of congestion,but the parameter is rarely used and is in fact deprecated (RFC6633, 2012) because suchfeatures are already present at the transport layer (Kurose & Ross, 2012, p. 353). Thereare other parameters that are also deprecated (IANA, 2013).Two types of query messages are prevalent today and can be used to check networkavailability and measure round-trip times. ICMP can therefore be used for testing,debugging and troubleshooting purposes, and is exactly what the popular programsping and traceroute (also tracert) rely upon for their functionality. ICMP messagescombined are very powerful and, aside from diagnostics, can even be used for maliciouspurposes such as mapping a network topology and probing for vulnerabilities. This is whysome systems either block these messages or do not report correctly (Kurose & Ross, 2012,p. 355). Like ARP, ICMP is a helper protocol of the network layer. It compensates forthe lack of error control and information reporting in IP, a deficiency which is unavoidabledue to the characteristics of a best-effort delivery service. ICMP is encapsulated withinIP datagrams so that only network-layer devices will act upon it.

5.2 Implementations

The ping program utilizes the ICMP echo-request and echo-reply pair of query messagesto obtain several pieces of information (Forouzan, 2009, p. 254; Kurose & Ross, 2012, p.353; Muuss, n.d.). It can report the round-trip packet transmission time between a hostand destination, and can indicate whether there is any problem along the path by keepinga count of packets sent and received. Proper operation of the networking hardware and IPsoftware is a precondition for the program to work (it is a networking utility), so it can beused to check network availability and functionality. Since a round-trip time measurementdepends on the response from the receiver, it can also be used to test network reachability.Because the program exposes IP addresses and can be used for malicious purposes, somesystems block ICMP messages due to privacy and security concerns (Kurose & Ross, 2012,p. 143, 355).The traceroute (or tracert) program is also based on ICMP, butrather interestinglyinstead of using query messages, relies on the time-exceeded and destination-unreachableerror-reporting messages to trace network hops along the path between a host anddestination (Forouzan, 2009, p. 260; Kurose & Ross, 2012, pp. 353355; RFC1393, 1993;Tanenbaum & Wetherall, 2010, p. 466). Such a route-recording option is also provided byping, but traceroute provides other options that offer more control (Forouzan, 2009, pp.

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204205). It works by sending out simple UDP datagrams and intentionally triggeringerrors incrementally, exploiting the behaviour of routers and end-hosts towards TTL valuesand invalid port numbers. Because the program also reveals a lot of information and canbe used to map out out the topology of a network, some systems block such messages andmay not respond (Kurose & Ross, 2012, p. 355).

5.3 Practical

The commands were run from a server located in Nuremburg, Germany. This informationcan be retrieved using IP geolocation services such as ipinfo.io. The CSU server locationwas also retrieved in this manner since the university and website may not necessarilybe colocated. Knowing the distance between the two servers is important in order todifferentiate delays, and this can be found by measuring by hand using a traditional mapor using an online map service such as Google Maps. The distance between Germany andAustralia is found to be approximately 15,000 km.

Figure 3: Ping from Germany to Australia

The ping command (Figure 3) was run with the options to prevent name-resolving (showonly IP addresses) and send 10 packets in order to reduce the output size to fit thispage. The results indicate that the CSU server is responding to the ICMP messages,and the latency (round-trip time) between the two servers is a constant 312 ms withoutany packet loss. This means that there are no variable delays introduced along thepath, such as congestion or queueing, although there can be fixed delays introduced byother factors. There is a minimum delay in this time, which is the propagation delay(distance propagation speed). The propagation speed cannot be faster than the speedof light, and typically in fiber optics (assuming trans-continental data communicationmedium is high-speed undersea fiber-optic cables) it is 2 108 m/s (Kurose & Ross, 2012,p. 37). Therefore:

Tp =15000 1032 108 = 0.075 = 75 ms

However, this is only in one direction, so the actual propagation delay included in thelatency between the two servers is 2 75 = 150 ms, leaving approximately another 150ms as fixed delay due to unaccountable factors. This also means that the propagationdelay is 50% of the packet delivery time.

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The traceroute command (Figure 4) was also run with options to suppress names andto send only one probe packet per hop in order to reduce the size of the image, therebyrecording only one timestamp. The output was also truncated from hop 19 onwards(up to 30) since they are simply non-response indicators (represented by the asteriskcharacter). It can be seen that 10th hop does not respond either, and this may be becauseICMP messages have been blocked. The incremental delays seen are likely only due togeographical distance between the hops (the ping run confirms that there are no variabledelays in the path). The first few hops are closer to Germany and the last few are closerto Australia. Because the trace does not end before the 30th hop and hops after the 18thare non-responsive, it can be said that the CSU server does not respond to traces. Thismay be for security reasons.

Figure 4: Traceroute from Germany to Australia

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References

Ars Technica. (2013). The telephone network is obsolete: get ready for the allIP telco. A technology news and information website. Retrieved Septem-ber 7, 2014, from http://arstechnica.com/information-technology/2013/01/the-telephone-network-is-obsolete-get-ready-for-the-all-ip-telco/

AT&T. (n.d.). History of network switching. The official website of AT&T Corp. RetrievedSeptember 7, 2014, from http://www.corp.att.com/history/nethistory/switching.html

COMNEWS. (1984). Electronic switching is in. Communications news: enterprisenetwork solutions, 21 (9), 8232.

Forouzan, B. A. (2006). Data communications and networking (4th ed.). New York,United States: McGraw-Hill.

Forouzan, B. A. (2009). TCP/IP protocol suite (4th ed.). New York, United States:McGraw-Hill.

Holzmann, G. J., & Pehrson, B. (1994). The early history of data networks. New York,United States: Wiley.

IANA. (2013). Internet control message protocol (ICMP) parameters. Official websiteof the Internet Assigned Numbers Authority. Retrieved September 7, 2014, fromhttp://www.iana.org/assignments/icmp-parameters/icmp-parameters.xhtml

Kurose, J. F., & Ross, K. W. (2012). Computer networking: a top-down approach (6thed.). New York, United States: Pearson.

Leiner, B. M., Cerf, V. G., Clark, D. D., Kahn, R. E., Kleinrock, L., Lynch, D. C., . . .Wolff, S. (2009). A brief history of the Internet. ACM SIGCOMM ComputerCommunications Review, 39 (5), 2231. Retrieved from http://www.internetsociety.org/internet/what-internet/history-internet/brief-history-internet

Muuss, M. (n.d.). The story of the PING program. US Army Research Laboratory userpage. Retrieved September 7, 2014, from http://ftp.arl.army.mil/~mike/ping.html

RFC1393. (1993). Traceroute using an IP option. The Internet Engineering Task Force.Retrieved from http://tools.ietf.org/html/rfc1393

RFC6633. (2012). Deprecation of ICMP source quench messages. The Internet Engineer-ing Task Force. Retrieved from http://tools.ietf.org/html/rfc6633

RFC792. (1981). Internet control message protocol. The Internet Engineering Task Force.Retrieved from http://tools.ietf.org/html/rfc792

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Roberts, L. G. (1978). The evolution of packet switching. Proceedings of the IEEE, 66 (11),13071313.

Tanenbaum, A. S., & Wetherall, D. J. (2010). Computer networks (5th ed.). New Jersey,United States: Prentice Hall.

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SwitchingPacket SwitchingCircuit SwitchingPackets vs. Circuits

SubnettingGroup 1First Customernth Customer

Group 2First Customernth Customer

Group 3First Customernth Customer

ForwardingRouter R1Router R2Router R3

ARPICMPThe ProtocolImplementationsPractical

References

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