Network Layer – IPv4 Addressing, Subnetting, CIDR, and Datagram Forwarding (Lecture Notes)

Network Layer – Lecture Notes (Week 4)

  • OSI Model context

    • The Network Layer sits above Data Link and Physical layers and below Transport in the OSI model.
    • Key functions at the Network Layer: routing (determining the path for packets) and forwarding (moving packets from a router’s input to the appropriate output port).
    • Practical analogy:
    • Routing: planning a trip from source to destination.
    • Forwarding: getting through a single interchange on that trip.
    • Core responsibilities per slide: end-to-end connections and data delivery, routing data packets, and selecting the best path to deliver data.

  • Two core Network Layer functions (overview)

    • Routing: determines the route taken by packets from source to destination – implemented via routing algorithms.
    • Forwarding: moves packets from the router’s input to the appropriate router’s output.
    • Analogy recap: routing = trip planning; forwarding = traversing a single leg.

  • Network Layer within the broader protocol stack (context)

    • End-to-end delivery is achieved through hops across multiple networks.
    • Routers make forwarding decisions using routing tables and IP addressing.
    • The Network Layer is responsible for selecting routes and ensuring data reaches the destination across multiple networks; the Data Link and Physical layers handle actual transmission on a given link.

  • IP addressing fundamentals (IPv4) – 32-bit addressing and dotted decimal notation

    • An IP address is a 32-bit binary number.
    • Readability format: divide into four 8-bit octets and convert each to decimal, e.g. the 32-bit binary example can be grouped into 4 octets and shown as dotted decimal: a.b.c.da.b.c.d
    • Example workflow from slide:
    • Binary example: 10101100 00010000 10000000 00010001
    • Decimal per octet: 172, 16, 128, 17
    • Dotted decimal notation: 172.16.128.17172.16.128.17
    • Addresses can be written in binary, decimal, or dotted decimal formats; networks rely on consistent representation for routing and subnetting.

  • IP Address Classes (overview) and role

    • Class A, B, C are the most commonly referenced in traditional (classful) addressing:
    • Class A: large number of hosts per network; default network prefix /8.
    • Class B: medium size; default prefix /16.
    • Class C: smaller networks; default prefix /24.
    • Class D: multicast addresses (not commonly used for unicast host addressing).
    • Class E: experimental addresses (not normally assigned publicly).
    • Addresses also include special-purpose ranges (e.g., 127.0.0.1 is loopback).
    • Loopback note: 127.0.0.0/8 reserved for loopback testing and cannot be assigned to devices on a network.

  • Subnetting: subnet concept and purpose

    • A subnet is a network within a network; subnetting divides a larger network into two or more smaller networks.
    • Benefits of subnetting:
    • Easier network management.
    • Reduced overall traffic within each subnet.
    • Easier application of network security policies.
    • Subnet mask concept: separates the IP address into a network part and a host part.
    • In the subnet mask, network bits are 1s and host bits are 0s.
    • Bitwise AND of IP address and subnet mask yields the network prefix.
    • Subnet mask defines how many bits are significant for the network portion.

  • Subnetting examples and key ideas

    • Subnetting example (conceptual): a given IP address and a subnet mask produce a network prefix and a host ID.
    • A concrete example shown: subnet 192.168.5.0/24
    • IP address: 192.168.5.130 with subnet mask 255.255.255.0 (i.e., /24).
    • Network prefix: 192.168.5.0
    • Host part: 130 (binary 00000000.00000000.00000000.10000010 in host bits)
    • In the /24 example, the first 24 bits are the network, last 8 bits identify hosts within that network.
    • The subnet mask can be represented in binary form to illustrate the boundary: 11111111.11111111.11111111.00000000 for /24.

  • CIDR (Classless Inter-Domain Routing) fundamentals

    • CIDR purpose: improve IP address allocation efficiency and delay IPv4 address exhaustion.
    • CIDR notation: a/b where a is an IP address and b is the number of network bits in the prefix (the network mask length).
    • IPv4 address space: 2^32 ≈ 4.29 billion addresses.
    • Classful addressing (A, B, C) allocates in 8-bit increments and can be wasteful (minimum allocation of 256 addresses for many organizations).
    • CIDR allows allocation in smaller (or larger) blocks by prefix length, e.g. /8, /16, /24, etc., enabling precise control over address space.
    • CIDR can represent classful addresses by using the corresponding prefix length (e.g., Class A = /8, Class B = /16, Class C = /24).
    • CIDR notational example: 216.3.128.0/25 represents a block with a 25-bit network prefix.
    • CIDR RFC reference: RFC 1519 (for detailed CIDR specs).

  • CIDR example calculations and practical blocks

    • Example: 216.3.128.0/25
    • Network prefix length: 25 bits.
    • Host bits: 32 - 25 = 7 bits.
    • Possible hosts per subnet: 27−2=1262^{7} - 2 = 126
    • Subnet mask in dotted decimal: 255.255.255.128255.255.255.128
    • Example: ISP block 122.211.0.0/16
    • Network prefix length: 16 bits; host bits: 16.
    • A customer needing up to 6 hosts can be allocated a /29 block if appropriate: 122.211.176.208/29.
    • Subnet mask for /29: 255.255.255.248255.255.255.248 (3 bits for host part).

  • Subnetting in detail: number of subnets and hosts (Classful context)

    • Class C networks:
    • Default: /24 (network bits = 24; host bits = 8).
    • If you borrow s bits for subnets from the host portion, then:
      • Number of subnets: Nsubnets=2sN_{subnets} = 2^{s}
      • Number of hosts per subnet: Nhosts=28−s−2N_{hosts} = 2^{8 - s} - 2
    • Class B networks:
    • Default: /16 (host bits = 16).
    • If you borrow s bits from hosts for subnets:
      • Subnets: Nsubnets=2sN_{subnets} = 2^{s}
      • Hosts per subnet: Nhosts=216−s−2N_{hosts} = 2^{16 - s} - 2
    • Class A networks:
    • Default: /8 (host bits = 24).
    • If you borrow s bits:
      • Subnets: Nsubnets=2sN_{subnets} = 2^{s}
      • Hosts per subnet: Nhosts=224−s−2N_{hosts} = 2^{24 - s} - 2
    • Practical note: traditional tables show subnets per class and available hosts per subnet; modern practice often uses CIDR rather than strict classful borrowing.

  • CIDR-based address allocation implications

    • CIDR enables flexible aggregation (route summarization) and avoids excessive subnetting waste.
    • The prefix length (b) indicates the number of network bits; host bits are 32 - b.
    • Example recap:
    • /25: 7 host bits → up to 126 usable hosts per subnet.
    • /29: 3 host bits → up to 6 usable hosts per subnet.

  • IPv4 vs IPv6 (high-level differences)

    • IP address size:
    • IPv4: 32-bit addresses.
    • IPv6: 128-bit addresses.
    • Address space:
    • IPv4: ~4.3 billion addresses (2^32).
    • IPv6: ~3.4 × 10^14 (340 trillion) addresses (2^128).
    • Notation and formatting:
    • IPv4: four groups of two hexadecimal digits separated by dots (example: 172.16.50.3).
    • IPv6: eight groups of four hexadecimal digits separated by colons (example: 2001:0db8:0012:0001:3c5e:7354:0000:5db1).
    • (Note: Both protocols will coexist until IPv6 fully replaces IPv4; IPv6 adoption has been increasing over time; as of late 2022, IPv6 share of Internet traffic was significant according to Google statistics.)

  • Datagram forwarding and routing (how a datagram gets from source to destination)

    • IP datagram carries source IP, destination IP, and payload, and is routed hop-by-hop across networks.
    • Forwarding table concept (illustrative): destination network, next router, and number of hops to next hop.
    • Key steps in delivering a datagram:
    • At the source, determine the destination network address.
    • If the destination is on the same network, the data can be delivered directly via the data link layer to the destination.
    • If the destination is on a different network, forward the datagram to the next-hop router according to the routing table.
    • The datagram then travels from router to router until it reaches the destination network, where the final delivery occurs on the destination link.
    • Illustration examples (high level):
    • Case A: destination on the same network as the source → direct link-layer delivery within the local network.
    • Case E: destination on a different network → router-by-router forwarding using the next-hop information in routing tables.
    • Routing decisions are based on the destination IP, routing tables, and network topology; forwarding is the act of moving a datagram between interfaces on a router.

  • Practical network design and learning checkpoints (course context)

    • Week 4 focus includes network layer devices and configurations, subnetting, and CIDR.
    • Practical lab activities include designing a network in Cisco Packet Tracer and configuring routers and PCs to reinforce subnetting concepts and routing/forwarding fundamentals.
    • Reading and reference materials include classic textbooks and online resources on OSI layers, subnetting, and IP addressing.

  • Key formulas and reference calculations (summary)

    • Subnetting host capacity (classful baseline):
    • Class A: hosts per subnet when s bits borrowed: Nhosts=224−s−2N_{hosts} = 2^{24 - s} - 2
    • Class B: Nhosts=216−s−2N_{hosts} = 2^{16 - s} - 2
    • Class C: Nhosts=28−s−2N_{hosts} = 2^{8 - s} - 2
    • Number of subnets when borrowing s bits:
    • Nsubnets=2sN_{subnets} = 2^{s}
    • CIDR host capacity for a given prefix length b (network bits):
    • Hosts per subnet: Nhosts=232−b−2N_{hosts} = 2^{32 - b} - 2
    • CIDR examples:
    • For a /25 network: Nhosts=232−25−2=27−2=126N_{hosts} = 2^{32 - 25} - 2 = 2^{7} - 2 = 126
    • For a /29 network: Nhosts=232−29−2=23−2=6N_{hosts} = 2^{32 - 29} - 2 = 2^{3} - 2 = 6
    • IPv4 vs IPv6 scope notes:
    • IPv4 address space: 232=4,294,967,2962^{32} = 4{,}294{,}967{,}296
    • IPv6 address space: 2128extaddressesext(verylarge, 3.4×1038)2^{128} ext{ addresses} ext{ (very large, ~3.4×10^{38})}

  • References and suggested readings (from slides)

    • Tanenbaum, A., Wetherall, D., Computer Networks, 5/E, Prentice Hall
    • Comer, D., Computer Networks and Internets, 5/E, Prentice Hall
    • Paterson, D.A., Hennessy, J.L., Computer Organization and Design, 4th Ediion
    • Kurose, J. Ross, K., Computer Networking: A Top-Down Approach
    • Computer Networking Notes and online study guides: e.g., CCNA study guide resources

  • Quick recap of the week’s lab and tasks

    • In this week’s lab, you will design a network in Cisco Packet Tracer, configure routers and PCs, and reinforce subnetting learning.

  • Notes on practical application and real-world relevance

    • CIDR is essential for efficient IP address management and scalable routing on the Internet.
    • Subnetting allows organizations to control broadcast domains, implement security policies, and tailor network size to departmental needs.
    • Understanding how datagrams are forwarded hop-by-hop helps in diagnosing routing issues and optimizing network performance.

  • Concepts to remember for exams

    • Distinguish routing vs forwarding and give examples.
    • Be able to convert between binary and dotted decimal and to segment an IP address into network and host portions given a subnet mask.
    • Compute the number of subnets and hosts given a classful network with borrowed bits, and perform similar calculations for CIDR-prefix networks.
    • Explain the differences between IPv4 and IPv6 in terms of address size, formatting, and the rationale for CIDR.
    • Describe how a datagram is routed from source to destination across multiple routers using routing tables and next-hop information.