Synetic College Logo
Unit 3 of 3 · M.Sc IT Sem 2

Unit 3: Internet protocols and network security

Computer Networks notes · PTU syllabus (PGCA1910)

3 min read12 topics10 exam questions
On this page
  1. Unit summary
  2. Principles of internetworking
  3. Connectionless internetworking
  4. The Internet Protocol (IPv4)
  5. IPv6
  6. Security requirements and attacks
  7. Encryption
  8. Conventional and public key encryption
  9. The RSA algorithm
  10. Digital signatures
  11. Distributed applications: SNMP
  12. Distributed applications: SMTP
  13. Distributed applications: HTTP
  14. Key terms
  15. Quick revision
  16. Important questions

Unit summary

The internet joins thousands of networks, must be secured, and supports everyday applications. This unit covers the principles of internetworking, connectionless internetworking, the Internet Protocol and IPv6, security requirements and attacks, encryption, public key encryption and digital signatures, and distributed applications — SNMP, SMTP and HTTP.

After this unit you can

  • Explain the principles of internetworking
  • Describe IPv4 and IPv6
  • Explain security requirements, attacks and cryptographic protection
  • Explain SNMP, SMTP and HTTP

PTU syllabus topics

  • Principles of internetworking
  • connectionless internetworking
  • Internet protocols and IPv6
  • security requirements and attacks
  • encryption
  • public key encryption and digital signatures
  • distributed applications — SNMP
  • SMTP
  • HTTP
ClassificationSecurity goals and attacks
Network security

  • Confidentiality

    Threat: eavesdropping

  • Integrity

    Threat: modification

  • Authentication

    Threat: spoofing

  • Availability

    Threat: denial of service

  • Non-repudiation

    Digital signatures

1

Topic 1

Principles of internetworking

  • Internetworking: connecting different networks (LANs, WANs with different protocols) so they work as one — the internet is a network of networks.
Key termsInternetworking devices
Repeater or hub
Physical layer; extends a segment
Bridge or switch
Data link layer; connects LAN segments
Router
Network layer; connects different networks, chooses routes
Gateway
Up to the application layer; translates between different protocols
  • Issues: different addressing, packet sizes (fragmentation), quality of service and security; solved by IP as a common protocol, tunnelling and fragmentation.
Key termsInternetworking requirements
Link between networks
Physical and link-layer connection
Routing and delivery
Across multiple networks
Accounting and status
Track use and keep status information
Hide differences
Addressing, maximum packet size, access mechanisms, timeouts, error recovery, status reporting, routing, connection or connectionless service
2

Topic 2

Connectionless internetworking

  • The internet is a datagram (connectionless) internetwork: each IP packet carries full source and destination addresses and is routed independently; routers keep no per-connection state.
  • Advantages: flexible, robust when routers fail, simple routers. Disadvantages: no delivery guarantee, packets may arrive out of order — reliability is added by TCP at the hosts.
  • Fragmentation: a router splits a datagram that exceeds the next network's MTU; the destination reassembles fragments using identification, flags and fragment offset fields.
3

Topic 3

The Internet Protocol (IPv4)

Key termsIPv4 header fields
Version and header length
4 and 20–60 bytes
Type of service (DSCP)
Priority and quality of service
Total length
Up to 65,535 bytes
Identification, flags, fragment offset
Fragmentation and reassembly
Time to live (TTL)
Hop limit to stop looping packets
Protocol
Upper layer — 6 TCP, 17 UDP
Header checksum
Error check of the header
Source and destination addresses
32 bits each
  • Addressing: classes A, B, C (now replaced by CIDR, e.g., 192.168.10.0/24 has 256 addresses), private ranges with NAT, and supporting protocols ICMP (ping, errors), ARP (IP to MAC) and DHCP (automatic configuration).
4

Topic 4

IPv6

ComparisonIPv4 and IPv6
IPv4
IPv6

Address size

32 bits (about 4.3 billion)

128 bits (about 3.4 × 10³⁸)

Notation

Dotted decimal — 172.16.4.1

Hexadecimal groups — 2001:db8:0:1::25

Header

Variable, 20–60 bytes, with checksum

Fixed 40 bytes with extension headers; no checksum

Fragmentation

By routers and hosts

Only by the source host

Configuration

Manual or DHCP

Stateless autoconfiguration (SLAAC) or DHCPv6

Security

IPsec optional

IPsec designed in

Broadcast

Yes

No — multicast and anycast instead

  • Transition mechanisms: dual stack, tunnelling (IPv6 inside IPv4) and translation (NAT64).
5

Topic 5

Security requirements and attacks

FrameworkSecurity requirements

  • Confidentiality

    Only authorised parties read data

  • Integrity

    Data not altered without detection

  • Availability

    Services usable when needed

  • Authenticity and non-repudiation

    Verify identities; senders cannot deny messages

ComparisonPassive and active attacks
Passive attacks
Active attacks

Nature

Eavesdropping without changing data

Modify, fabricate or block data

Types

Release of message contents, traffic analysis

Masquerade, replay, modification of messages, denial of service

Detection

Hard — prevent with encryption

Easier to detect; aim to recover

6

Topic 6

Encryption

ProcessEncryption model
  1. 1Plaintext

    Readable message

  2. 2Encryption algorithm with key
  3. 3Ciphertext

    Unreadable message sent

  4. 4Decryption algorithm with key
  5. 5Plaintext recovered
Key termsCryptography terms
Cryptography
Science of secret writing
Cryptanalysis
Breaking ciphers without the key
Cryptology
Cryptography and cryptanalysis together
Key
Secret value controlling encryption
Brute-force attack
Trying every possible key
7

Topic 7

Conventional and public key encryption

ComparisonSymmetric vs asymmetric encryption
Symmetric (secret key)
Asymmetric (public key)

Keys

One shared secret key

Pair — public key and private key

Speed

Fast

Slower

Key distribution

Difficult — key must be shared securely

Easy — public key can be published

Examples

AES, DES, 3DES

RSA, ECC

Use

Bulk data encryption

Key exchange, digital signatures

  • Hybrid approach (SSL/TLS): asymmetric encryption to exchange a session key, then symmetric encryption for data.
  • Public/private key pair: what one key encrypts only the other can decrypt — encrypt with the receiver's public key for confidentiality; sign with the sender's private key for authentication.
  • Hash functions (SHA-256) produce fixed-length digests for integrity checks; AES is the common symmetric standard; RSA and elliptic-curve cryptography are common asymmetric methods.
8

Topic 8

The RSA algorithm

ProcessRSA key generation, encryption and decryption
  1. 1

    Choose two large primes p and q

  2. 2

    Compute n = p × q and φ(n) = (p − 1)(q − 1)

  3. 3

    Choose e with 1 < e < φ(n) and gcd(e, φ(n)) = 1

  4. 4

    Compute d such that d × e mod φ(n) = 1

  5. 5

    Public key (e, n); private key (d, n)

  6. 6

    Encrypt C = M^e mod n; decrypt M = C^d mod n

Example

p = 3, q = 11 → n = 33, φ(n) = 20. Choose e = 3 (gcd(3, 20) = 1). d = 7, since 3 × 7 = 21 mod 20 = 1. Encrypt M = 4: C = 4³ mod 33 = 64 mod 33 = 31. Decrypt: 31⁷ mod 33 = 4.

  • Security: rests on the difficulty of factoring n; keys of 2048 bits or more are recommended.
9

Topic 9

Digital signatures

  • Digital signature: an electronic signature using asymmetric cryptography and a hash function to authenticate the signer and ensure integrity.
ProcessSigning and verifying
  1. 1Hash the document
  2. 2Encrypt the hash with the signer's private key
  3. 3Send document, signature and certificate
  4. 4Receiver decrypts the signature with the public key
  5. 5Compare with a fresh hash — match means authentic and unaltered
  • Digital Signature Certificate (DSC): classes based on verification level (Class 3 and document signer certificates now in use); used for MCA filings, income-tax returns, GST, e-tenders.
  • Aadhaar e-Sign: online electronic signature using Aadhaar OTP or biometric authentication.
  • Properties: authentication of the signer, integrity of the message, non-repudiation. Standards: RSA signatures, DSA (Digital Signature Standard), ECDSA; legally valid in India under the IT Act, 2000.
10

Topic 10

Distributed applications: SNMP

  • Simple Network Management Protocol monitors and manages devices (routers, switches, servers, printers) over UDP (ports 161 and 162).
Key termsSNMP components
Manager (NMS)
Station that polls and controls devices
Agent
Software on each managed device
MIB
Management information base — variables such as interface counters, identified by object IDs
Operations
Get, GetNext, GetBulk, Set, Trap (unsolicited alert), Inform
Versions
v1 and v2c use community strings; v3 adds authentication and encryption
11

Topic 11

Distributed applications: SMTP

ProcessEmail delivery
  1. 1User agent (Outlook, Gmail) composes mail
  2. 2Submission to the sender's mail server (port 587)
  3. 3SMTP transfer between mail servers (port 25) after a DNS MX lookup
  4. 4Stored in the recipient's mailbox
  5. 5Recipient retrieves it with POP3 or IMAP
  • SMTP commands: HELO or EHLO, MAIL FROM, RCPT TO, DATA, QUIT; MIME extends email to attachments and non-ASCII text.
12

Topic 12

Distributed applications: HTTP

  • HyperText Transfer Protocol: the request–response protocol of the web, over TCP port 80 (HTTPS on 443 with TLS); stateless — cookies and sessions add state.
Key termsHTTP essentials
Methods
GET (read), POST (submit), PUT (replace), DELETE (remove), HEAD, PATCH
Status codes
200 OK, 301 moved, 304 not modified, 404 not found, 500 server error
Headers
Host, Content-Type, Cache-Control, Set-Cookie
Versions
HTTP/1.1 persistent connections; HTTP/2 multiplexing; HTTP/3 over QUIC (UDP)

Key terms

Internetworking
Connecting different networks into one
Datagram
Independently routed packet carrying full addresses
IPv6
128-bit version of the Internet Protocol
Active attack
Attack that modifies or disrupts data
SNMP
Protocol for managing network devices

Quick revision

  • Internetworking requirements; routers and gateways; connectionless datagrams; fragmentation.
  • IPv4 header; CIDR; ICMP, ARP, DHCP.
  • IPv6 features; comparison; transition methods.
  • CIA; passive vs active attacks; symmetric and public key encryption; RSA; digital signatures.
  • SNMP manager, agent, MIB; SMTP with POP3 and IMAP; HTTP methods, status codes, versions.

Important exam questions

Practice questions written to the PTU exam pattern for this unit's syllabus: short answers (Section A style) and long answers (Sections B and C style).

Short-answer questions

  1. Q1.What is connectionless internetworking?
  2. Q2.What is the purpose of the TTL field?
  3. Q3.State three advantages of IPv6 over IPv4.
  4. Q4.Distinguish passive and active attacks.
  5. Q5.What is a MIB?
  6. Q6.Distinguish GET and POST.

Long-answer questions

  1. Q1.Explain the principles of internetworking and connectionless internetworking.
  2. Q2.Compare IPv4 and IPv6.
  3. Q3.Explain security requirements, attacks, encryption and digital signatures.
  4. Q4.Explain the distributed applications SNMP, SMTP and HTTP.

Stuck on this unit?

Message SBS on WhatsApp for help with Computer Networks, or to ask about studying M.Sc IT at Synetic.

WhatsApp us