Wireless & Mobile Computing (CS-702 (C)) Unit 1 Notes

How unit 1 is examined

Revision of LAN/WAN, devices, the TCP/IP stack, IP addressing, subnetting, CIDR, NAT, VLANs and IPv6; the marks sit in TCP/IP architecture, subnetting and the IPv4 versus IPv6 comparisons.

Review of LAN, MAN, WAN, Intranet, Internet

Low weight

Definition. A LAN covers a building or campus, a MAN covers a city, and a WAN covers countries or continents; the Internet is the global network of networks, and an intranet is a private network that uses Internet technology.

Key points.

A LAN (Local Area Network) spans up to a few km, is privately owned, uses Ethernet or Wi-Fi at 100 Mbps to 10 Gbps, and is cheap with very low delay.
A MAN (Metropolitan Area Network) spans a city (about 5-50 km), is owned by an operator or city body, uses fibre or WiMAX, and gives medium speed, as in cable TV and campus-to-campus links.
A WAN (Wide Area Network) spans countries, uses leased lines, satellite or MPLS through carriers, has the highest cost and delay and the lowest speed per rupee, and the Internet is its biggest example.
An intranet is a private organisation-only network that uses TCP/IP and web tools; an extranet opens part of it to partners.
Advantages of networking: resource sharing (files, printers), fast communication (email), lower cost and central backup.
Disadvantages of networking: security threats, setup complexity, maintenance cost, and total dependence on the network.

Basis	LAN	MAN	WAN
Range	Building or campus	City	Country or world
Speed	High	Medium	Lower
Cost	Low	Medium	High
Owner	One organisation	Operator or group	Many carriers
Example	College lab	Cable TV network	Internet

Answer frame. Open with the three definitions; draw the comparison table; then advantages and disadvantages as two short lists; close by noting the Internet is a WAN of WANs.

Asked: [7 marks] (Jun 2025) What is difference between LAN, MAN and WAN? Discuss advantages and disadvantages of networking.

Interconnectivity devices: bridges, routers

Definition. The Internet is the global network of networks that communicate using TCP/IP; devices such as repeaters, bridges, switches, routers and gateways join the networks together.

Key points.

A repeater or hub works at the physical layer and only regenerates the signal.
A bridge works at the data-link layer, learns MAC addresses, and filters or forwards frames so it splits collision domains.
A router works at the network layer, uses IP addresses and a routing table to pick the best path between different networks.
A gateway connects networks with different protocols and can work up to the application layer.

Basis	Bridge	Router
Layer	Data link (2)	Network (3)
Address used	MAC	IP
Broadcast domain	Forwards broadcasts, one domain	Blocks broadcasts, splits domains
Use	Join LAN segments of the same network	Join different networks, path selection

Asked: [7 marks] (Dec 2024) What do you mean by the term internet? Differentiate the operation of bridges and routers.

Review of TCP/IP Protocol Architecture

Definition. TCP/IP is a four-layer protocol suite (Application, Transport, Internet, Network Access) built on packet switching, in which each layer serves the one above it and the end hosts, not the network, ensure reliable delivery.

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Key points.

The basic design principle is packet switching with an end-to-end approach: the network only forwards datagrams on a best-effort basis, and the end hosts add reliability and robustness.
The Application layer gives user services through protocols such as HTTP, FTP, SMTP and DNS.
The Transport layer gives process-to-process delivery using port numbers, with segmentation and reassembly; TCP is reliable and connection-oriented, UDP is fast and connectionless.
The Internet layer gives logical IP addressing and routing of datagrams across networks, using IP, ICMP and ARP.
The Network Access layer sends frames over the physical medium using MAC addresses, for example Ethernet and Wi-Fi.
Encapsulation: data becomes a segment at Transport, a datagram at Internet and a frame at Network Access, and each receiving layer strips its own header.
Transport is end-to-end because only the two hosts run TCP/UDP: routers in between look only at the IP header. Example: in an FTP file transfer TCP splits the file into segments, numbers them, the receiver acknowledges and reorders them, and port 21 finds the right application; for live video UDP is chosen because speed matters more than lost packets.

Answer frame. Open with the definition and the design principle; draw the four-layer stack with protocols and an arrow for encapsulation; develop points 2-5 layer by layer; for the transport question spend the answer on points 3, 6 and 7 with the FTP example; close with a line that transport gives end-to-end delivery.

Asked: [7 marks] (Dec 2024, Jun 2025) Draw and explain TCP-IP architecture of a network. What is the basic principle involved in building up TCP/IP? Also discuss its protocol architecture. Asked: [7 marks] (Jun 2025) "In TCP/IP reference model, transport layer is responsible for end-to-end delivery". Justify the statement with a suitable example.

ARP/RARP

Definition. ARP maps a known IP address to a MAC address; RARP does the reverse, letting a diskless host find its own IP address from its MAC address.

Key points.

In RARP the booting host broadcasts a request carrying its MAC address, and a RARP server replies with the IP address.
Limitations: it uses link-level broadcast so routers do not forward it, so a RARP server is needed on every network.
It returns only the IP address, with no subnet mask, gateway or DNS, and it does not support subnetting.
Alternatives: BOOTP (works across routers through relay agents) and DHCP (dynamic leases plus mask, gateway and DNS); ICMP address-mask messages give a little extra information.
DHCP replaced RARP because it is automatic, flexible and routable.

Asked: [7 marks] (Jun 2025) What is RARP? Write its limitations. What are the alternative solutions to RARP?

IP addressing

Definition. An IPv4 address is a 32-bit logical address written in dotted decimal that is split into a network part and a host part.

Formula. Networks $=2^{n}$ for $n$ network bits; hosts per network $=2^{h}-2$ for $h$ host bits (all-0 is the network address and all-1 the broadcast).

Class	First bits	Network bits	Networks	Host bits	Hosts per network
A (1-126)	0	7	$2^{7}=128$ (126 usable)	24	$2^{24}-2=16{,}777{,}214$
B (128-191)	10	14	$2^{14}=16{,}384$	16	$2^{16}-2=65{,}534$
C (192-223)	110	21	$2^{21}=2{,}097{,}152$	8	$2^{8}-2=254$

Class D (224-239) is multicast and class E (240-255) is reserved.

Asked: [7 marks] (Dec 2020) How many class A, B and C network can exist? How many hosts can a network in each class have?

IP datagram format and its delivery

Definition. An IP datagram is the packet of IPv4, a header of 20-60 bytes followed by data.

Key points.

Header fields include version (4), header length, type of service, total length, identification, flags and fragment offset (for fragmentation), TTL, protocol, header checksum, source and destination addresses, and options.
Delivery is connectionless and best effort; direct delivery happens when the destination is on the same network, indirect delivery goes through a router.
Routers decrement TTL, recompute the checksum and fragment if the MTU is smaller.

Routing table format

Definition. A routing table is the list a router consults to choose the next hop for a destination.

Key points.

Each entry has destination network, subnet mask, next-hop (gateway), interface and metric (cost).
The router picks the longest matching prefix, and a default route (0.0.0.0/0) is used when nothing matches.
Entries are static (manual) or dynamic (learned from routing protocols).

ICMP messages

Definition. ICMP is the Internet layer protocol that reports errors and gives diagnostic information about IP delivery.

Key points.

Error messages: destination unreachable, time exceeded (TTL zero), source quench and redirect, parameter problem.
Query messages: echo request and reply (used by ping), and timestamp or mask requests.
Traceroute uses increasing TTL values and the time-exceeded replies to list the routers on a path.

Subnetting

Definition. Subnetting divides one network into smaller subnetworks by borrowing host bits to form a subnet field, and the subnet mask marks which bits are network and subnet bits.

Formula. Subnet address $=$ IP AND mask; broadcast $=$ subnet address with all host bits set to 1; with $b$ borrowed bits, subnets $=2^{b}$ and hosts per subnet $=2^{h}-2$.

Steps.

Step 1: Identify the class from the first octet.
Step 2: Decide how many subnets are needed and borrow b bits, where 2^b >= subnets.
Step 3: Write the new mask (network bits and borrowed bits set to 1).
Step 4: AND the IP address with the mask to get the subnet address.
Step 5: Set all remaining host bits to 1 to get the broadcast address.

Example. The given mask 253.295.295.128 is invalid because an octet cannot exceed 255, so it is taken as the intended 255.255.255.128 (/25).

Item	Working	Result
Class	First octet 172 lies in 128-191	Class B
Mask	255.255.255.128, last octet 10000000	1 borrowed bit
Subnet	Last octet: 5 AND 128 = 0	172.16.13.0
Broadcast	Host bits (7) all 1: 0 + 127	172.16.13.127

Usable hosts are 172.16.13.1 to 172.16.13.126, so $2^{7}-2=126$.

Answer frame. For the numerical, state the invalid-mask assumption first, then class, ANDing and broadcast in a table; for the "process" question define subnetting, give the steps with this example, then define supernetting (see the next topic) and end with the comparison.

Pitfall: Quote the mask correctly: writing 253.295.295.128 as valid loses marks; say octets are at most 255.

Asked: [7 marks] (Dec 2020) You have an IP Address of 172.16.13.5 with 253.295.295.128 subnet mask. What is your class of address, subnet address, and Broadcast address. Asked: [7 marks] (Dec 2024) Discuss the process of subnetting and supernetting.

Supernetting and CIDR

Definition. Supernetting combines several contiguous class C (or other) networks into one larger network, and CIDR (Classless Inter-Domain Routing) writes addresses as a.b.c.d/n where n is the prefix length in bits.

Key points.

Subnetting borrows host bits to make more, smaller networks; supernetting borrows network bits to make one bigger block, so it shortens the prefix (for example four /24 blocks aggregate to one /22).
CIDR ignores class boundaries and allows variable-length prefixes (VLSM), so the size of a block is $2^{32-n}$ addresses.
Advantages over classful addressing: less address waste, since a block of the right size is given; smaller routing tables through route aggregation; and slower exhaustion of IPv4.
Classful addressing wastes addresses (a class B for 500 hosts wastes 65,000) and bloats the routing tables.

Asked: [7 marks] (Dec 2020) What is CIDR? What are the advantages of CIDR over classful addressing?

DNS

Definition. DNS (Domain Name System) is the distributed database that translates host names into IP addresses.

Key points.

The namespace is a hierarchy: root, top-level domains (.com, .in), second-level domains and hosts.
A resolver asks the local name server, which queries root, TLD and authoritative servers, and caches the answers.
DNS uses UDP port 53 for queries and TCP for zone transfers.

NAT: private addressing and NAT

Definition. NAT (Network Address Translation) maps private addresses to public addresses at a router so many hosts can share one public IP.

Key points.

Private ranges are 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16, and they are not routed on the Internet.
NAT saves public IPv4 addresses and hides the internal network.
PAT (NAT overload) distinguishes hosts by port number.

SNAT

Definition. SNAT (source NAT) rewrites the source address of outgoing packets to a public address.

Key points.

It lets inside hosts reach the Internet, and the reply is translated back using the NAT table.
Masquerading is SNAT with a dynamic public address.
It is applied after routing (POSTROUTING).

DNAT

Definition. DNAT (destination NAT) rewrites the destination address of incoming packets to an internal host.

Key points.

It is used for port forwarding, so an outside client can reach an inside web server on a public IP and port.
It is applied before routing (PREROUTING).
It exposes an internal service without giving it a public address.

NAT and firewalls

Definition. A firewall is a security barrier that filters traffic by rules; NAT works beside it, since NAT hides internal addresses but is not itself security.

Key points.

By default NAT drops unsolicited inbound traffic because no mapping exists, which acts like a basic stateful firewall.
NAT breaks end-to-end addressing, and protocols that carry IPs inside data (FTP, SIP) need helpers.
Hole punching and port-forwarding rules allow inbound connections through both.

VLANs: concepts

Definition. A VLAN is a logical group of devices in one or more physical LANs that behave as if on one broadcast domain, regardless of their location; a firewall is a network security barrier that allows or blocks traffic according to rules.

Key points.

A VLAN is configured in software on a switch, so moving a user between groups needs no rewiring.
Each VLAN is its own broadcast domain, so broadcasts are contained, performance improves and security is better.
Traffic between VLANs needs a router or layer-3 switch.
Firewalls are packet-filter, stateful or application-proxy, in hardware or software, and sit between a trusted network and an untrusted one; example, the college firewall blocks outside access to the lab VLAN.
Use case: separate VLANs for students, staff and servers.

Asked: [7 marks] (Dec 2024) Define the terms: i) VLAN ii) Firewall.

Comparison with real LANs

Definition. A VLAN is logical, a real LAN is physical.

Key points.

A real LAN is defined by cabling and switches; a VLAN is defined by configuration.
Changing membership of a real LAN means rewiring, of a VLAN just reconfiguring.
Both are one broadcast domain, but VLAN members may be on different switches.

Types of VLAN

Definition. VLANs are classified by how a device is assigned to them.

Key points.

Port-based (static): the switch port decides the VLAN.
MAC-based (dynamic): the device's MAC address decides.
Protocol-based and IP-subnet-based: the protocol or subnet decides.

Tagging

Definition. Tagging (IEEE 802.1Q) inserts a 4-byte tag into an Ethernet frame so VLAN membership is carried over trunk links.

Key points.

The tag has a 16-bit TPID (0x8100), 3-bit priority, 1-bit DEI and a 12-bit VLAN ID (1 to 4094).
A trunk port carries tagged frames of many VLANs; an access port carries untagged frames of one.
The switch strips the tag before delivery to an access port.

IPv6: address structure

Definition. An IPv6 address is 128 bits long, written as eight groups of four hex digits separated by colons.

Key points.

Example: 2001:0db8:0000:0000:0000:0000:0000:0001.
Leading zeros in a group can be dropped and one run of all-zero groups replaced by ::, so the example becomes 2001:db8::1.
Types are unicast, multicast and anycast; there is no broadcast.

IPv6: address space and header

Definition. IPv6 uses 128-bit addresses giving $2^{128}$ addresses against $2^{32}$ for IPv4, and a simple fixed 40-byte header.

Key points.

Address space: $2^{32}\approx 4.3\times10^{9}$ for IPv4 against $2^{128}\approx 3.4\times10^{38}$ for IPv6.
The IPv6 header has 8 fields: version, traffic class, flow label, payload length, next header, hop limit, source and destination address.
Fields common to both: version, payload/total length, TTL/hop limit, protocol/next header, source and destination address.
IPv6 drops header length, identification, flags, fragment offset, header checksum and options from the base header; options move to extension headers.
Only the source host fragments in IPv6, and IPSec is built in.

Basis	IPv4	IPv6
Address size	32 bits	128 bits
Notation	Dotted decimal (192.168.1.1)	Hexadecimal, colon separated
Space	$2^{32}$	$2^{128}$
Header	20-60 bytes, variable, 12 fields	40 bytes, fixed, 8 fields
Checksum	Yes	No
Fragmentation	Routers and source	Source only
Security	Optional IPSec	IPSec built in
Broadcast	Yes	No, uses multicast

Answer frame. For the address question, define both, give the table rows for size, notation and space, and close on the shortage of IPv4; for the header question, draw both headers as field lists, list common fields (point 3), then removed fields, and close that IPv6 is simpler and extensible.

Asked: [7 marks] (Dec 2024) Differentiate IPv4 and IPv6 based on address structure and address space. Asked: [7 marks] (Jun 2025) Compare and contrast the IPv4 and IPv6 header fields. Do they have any fields in common?

Last-minute revision

LAN is under a few km, MAN a city, WAN a country; the Internet is a network of networks.
A bridge works at layer 2 with MAC addresses; a router works at layer 3 with IP addresses and splits broadcast domains.
TCP/IP layers: Application, Transport, Internet, Network Access.
Encapsulation: data, segment, datagram, frame.
ARP maps IP to MAC; RARP maps MAC to IP and is replaced by BOOTP and DHCP.
Networks $=2^{n}$; hosts $=2^{h}-2$; Class A 126 networks, B 16,384, C 2,097,152.
172.16.13.5 with /25: class B, subnet 172.16.13.0, broadcast 172.16.13.127.
CIDR uses prefix a.b.c.d/n; it aggregates routes and saves addresses.
Private ranges: 10/8, 172.16/12, 192.168/16.
802.1Q tag is 4 bytes with a 12-bit VLAN ID.
IPv6 is 128 bits with a 40-byte header and no checksum.

Memory hooks

Layers "A T I N" (Application, Transport, Internet, Network access): "All Trains In Nagpur".
Bridge = MAC = layer 2; Router = IP = layer 3.
SNAT changes the Source going out; DNAT changes the Destination coming in.
Subnet borrows (small pieces); supernet merges (big piece).
IPv6 = 128 bits = 8 groups of 16 bits.

Coverage checklist

Review of LAN, MAN, WAN, Intranet, Internet: Q3.
interconnectivity devices: bridges, Routers etc.: Q12.
Review of TCP/IP Protocol Architecture: Q4, Q5.
ARP/RARP: Q1.
IP addressing: Q2.
IP Datagram format and its Delivery: no past questions.
Routing table format: no past questions.
ICMP Messages: no past questions.
Subnetting: Q6, Q7.
Supernetting and CIDR: Q8 (and the supernetting half of Q7).
DNS: no past questions.
NAT: Private addressing and NAT: no past questions.
SNAT: no past questions.
DNAT: no past questions.
NAT and firewalls: no past questions (firewall definition in Q9).
VLANS: Concepts: Q9.
Comparison with Real LANS: no past questions.
Type of VLAN: no past questions.
Tagging: no past questions.
IPV6: address structure: Q10.
address space and header: Q10, Q11.

Account

Wireless & Mobile Computing (CS-702 (C)) - Unit 1 Short Notes

How unit 1 is examined

Review of LAN, MAN, WAN, Intranet, Internet

Interconnectivity devices: bridges, routers

Review of TCP/IP Protocol Architecture

ARP/RARP

IP addressing

IP datagram format and its delivery

Routing table format

ICMP messages

Subnetting

Supernetting and CIDR

DNS

NAT: private addressing and NAT

SNAT

DNAT

NAT and firewalls

VLANs: concepts

Comparison with real LANs

Types of VLAN

Tagging

IPv6: address structure

IPv6: address space and header

Last-minute revision

Memory hooks

Coverage checklist