Classful addressing (A–E) and special IPv4 addresses

1. How many bits are in one octet of an IPv4 address, and what is its decimal range?
   Stuck? Peek at 3.1.1 IPv4 address structure, dotted decimal, net-id vs host-id

   • IPv4 address = 32 bits = 4 octets (0–255 each), total space = 2³².
   • Split: Net-ID (left n bits) + Host-ID (right h = 32 − n bits).
   • Identifies a network interface (routers have one IP per link).

   Read lesson 3.1.1

2. What is the 48-bit broadcast MAC address in Ethernet?
   Stuck? Peek at 2.4.1 Ethernet frame format, MAC address types, minimum and maximum frame sizes

   • Frame: preamble 7 + SFD 1 | dest 6 | src 6 | type/len 2 | data 46–1500 | CRC 4.
   • Frame 64–1518 bytes (preamble excluded); pad data to 46.
   • MTU = 1500 bytes.
   • First-byte LSB 1 → multicast; FF:…:FF → broadcast; else unicast.
   • 64 bytes = 512 bits = CSMA/CD slot time at 10 Mbps.

   Read lesson 2.4.1

3. What is the decimal value of the 8-bit binary number 11100000?
   Stuck? Peek at 0.2.2 Binary, decimal and hex; powers of 2

   • Binary places: 1, 2, 4, 8, 16, 32, 64, 128, …; MSB left, LSB right.
   • Hex: one digit = 4 bits; A–F = 10–15; group from the right.
   • k bits → 2ᵏ values, 0 … 2ᵏ − 1.
   • Bits needed for N values = smallest k with 2ᵏ ≥ N.
   • 2¹⁰ = 1024 · 2¹⁶ = 65,536 · 2²⁰ ≈ 1.05 M · 2³² ≈ 4.29 × 10⁹.

   Read lesson 0.2.2

When IPv4 was designed in 1981, engineers wanted every router to know the boundary between net-id and host-id instantly just by looking at the first few bits of the address itself. So they carved the 2³² space into five fixed-size classes (A, B, C, D, E).

Although modern networks have replaced fixed classes with flexible boundaries (3.1.4), GATE regularly tests class ranges, default masks, and the reserved special addresses inside every block.

In a network with 8 host bits (2⁸ = 256 addresses), why can you only assign 254 of those addresses to actual computers?

Because two extreme host-id patterns are reserved in every network: all-0s in the host part is the network address (identifies the network itself), and all-1s in the host part is the directed broadcast address (sends to all hosts on that network).

Leading bit pattern & first-octet ranges (Classes A–E)

The position of the first 0 bit in the first octet tells you the class:

• Class A starts with 0... → first octet 0–127 (half of the entire IPv4 space!). Net-id = 1 octet (8 bits), Host-id = 3 octets (24 bits).
• Class B starts with 10... → first octet 128–191 (1/4 of space). Net-id = 2 octets (16 bits), Host-id = 2 octets (16 bits).
• Class C starts with 110... → first octet 192–223 (1/8 of space). Net-id = 3 octets (24 bits), Host-id = 1 octet (8 bits).
• Class D starts with 1110... → first octet 224–239 (1/16 of space). Used for multicast (one-to-many group delivery; no net/host split).
• Class E starts with 1111... → first octet 240–255 (1/16 of space). Reserved / experimental.

Network address vs Directed Broadcast address

Inside any unicast network (Class A, B, or C) with h host bits:

Five special IPv4 addresses you must know for GATE

Several patterns have fixed meanings across the entire Internet:

Putting it together

• Classful addressing fixes the net/host boundary using the leading bits: A (0–127, 8 net), B (128–191, 16 net), C (192–223, 24 net), D (224–239, multicast), E (240–255, reserved).
• Every network reserves all-0s host bits as the network address and all-1s host bits as the directed broadcast, leaving 2ʰ − 2 usable hosts.
• 0.0.0.0 (this host), 255.255.255.255 (limited broadcast), and 127.x.y.z (loopback) have special rules tested frequently on GATE.

• 127.x.y.z is wasted! All 2²⁴ addresses starting with 127 (not just 127.0.0.1) are reserved for loopback, so Class A has 126 usable networks (1..126), not 128.
• Limited vs Directed Broadcast: 255.255.255.255 stops at the first router (local LAN only); 172.16.255.255 (Class B directed broadcast) can be routed to network 172.16.0.0 and then broadcast there.
• Why classful died: A company needing 300 hosts couldn't fit in Class C (254) and had to take a Class B (65,534), wasting over 65,200 addresses!

0, 128, 192, 224, 240 — keep halving the remaining pie (128, +64, +32, +16) for A, B, C, D, E!

Classful addressing wasted millions of addresses because a Class B network forced all 65,534 hosts into a single giant LAN. Next question (3.1.3): How can an organisation take one Class B block and slice it internally into dozens of smaller networks?

1. MCQ

   To which IPv4 class does the address 191.255.10.1 belong, and what is its default mask?

   1. Class A, 255.0.0.0
   2. Class B, 255.255.0.0
   3. Class C, 255.255.255.0
   4. Class D, no mask
2. NAT

   In a standard Class C network, how many usable host addresses are available?

   Write your answer before opening the key.

3. MSQ

   Which of the following IPv4 addresses can NEVER be used as the source IP address of a valid packet on a link?

   1. 224.0.1.10
   2. 255.255.255.255
   3. 192.168.1.0 (in a standard Class C network)
   4. 192.168.1.1 (in a standard Class C network)
4. MCQ

   What fraction of the total IPv4 address space is occupied by Class C addresses?

   1. 1/2 (50%)
   2. 1/4 (25%)
   3. 1/8 (12.5%)
   4. 1/16 (6.25%)
5. MCQ

   A host sends a packet to destination 127.15.20.1. What happens to the packet?

   1. It is broadcast on the local Ethernet.
   2. It is forwarded by the default router to network 127.
   3. It is looped back inside the sending host by the network layer and never transmitted on any link.
   4. It is dropped as an invalid Class E address.

• Classes by 1st octet: A = 0–127 (0, 8 net bits), B = 128–191 (10, 16 net bits), C = 192–223 (110, 24 net bits), D = 224–239 (1110, multicast), E = 240–255 (1111, reserved).
• Usable hosts = 2ʰ − 2 (exclude host all-0s = network address, host all-1s = directed broadcast).
• Specials: 0.0.0.0 = this host (src only); 255.255.255.255 = limited broadcast (local LAN only); 127.x.y.z = loopback.

• classful addressing
• classes A/B/C/D/E
• default mask
• network address
• directed broadcast
• limited broadcast (255.255.255.255)
• loopback (127.x.y.z)
• 0.0.0.0
• multicast (Class D)

1. First-octet ranges for Classes A, B, C, D, E?
   Reveal answer

   A: 0–127, B: 128–191, C: 192–223, D: 224–239, E: 240–255.
2. Why is the number of usable hosts 2ʰ − 2?
   Reveal answer

   Host-id all-0s is the network address; host-id all-1s is the directed broadcast address.
3. Difference between 255.255.255.255 and a directed broadcast?
   Reveal answer

   255.255.255.255 (limited broadcast) stays on the local LAN and is never forwarded by routers; a directed broadcast targets a specific network.
4. What happens to a packet sent to 127.0.0.1?
   Reveal answer

   It loops back inside the host’s own protocol stack and never touches the physical link.

• First-octet boundary ladder: add 128, 64, 32, 16 → starts of A, B, C, D, E are 0, 128, 192, 224, 240. — identifying the class of any dotted-decimal IPv4 address instantly.
• Source vs Destination legality: Broadcast and Multicast addresses can only be destinations; 0.0.0.0 can only be a source. — GATE MSQs asking "Which of the following can never be a valid source/destination IP?"