IPv4 — address format, classes, subnetting, CIDR notation

4.3.8 · Coding › Computer Networks

1. Address Format — ek IPv4 address actually HAI kya

WHY dotted decimal? 32 raw bits (11000000101010000000000100000001) padhne laayak nahi hote. Hum inhe 8-8 bits ke 4 groups mein todke har group ko decimal mein likhte hain. $192.168.0.1$ bas human-friendly packaging hai.

2. Network vs Host — har cheez ke peeche yahi key idea hai

Yeh split ek mask se define hoti hai: sabse baayein $n$ bits network ke hain, baaki host ke.

3. Classful Addressing — PURANA (rigid) split

Definition Classes (leading bits se define hoti hain)

Class	Leading bits	First octet	Network bits	Host bits	# hosts/net
A	`0`	1–126	8	24	$2^{24} - 2$
B	`10`	128–191	16	16	$2^{16} - 2$
C	`110`	192–223	24	8	$2^{8} - 2$
D (multicast)	`1110`	224–239	—	—	—
E (reserved)	`1111`	240–255	—	—	—

WHY " $- 2$ "? Kisi bhi network mein all-zeros host = network address khud, aur all-ones host = broadcast address. Dono mein se kisi ko bhi real machine ko assign nahi kiya ja sakta.

4. Subnetting — host bits borrow karna

Figure — IPv4 — address format, classes, subnetting, CIDR notation

5. CIDR Notation — modern, classless system

6. Reserved / special ranges (yeh yaad rakho)

Common mistake Common traps (Steel-man + fix)

Trap 1: "Ek /26 mein 64 usable hosts hote hain." Yeh sahi kyun lagta hai: $2^{6} = 64$ , aur hamare paas 6 host bits hain. Fix: 2 subtract karna zaroori hai (network + broadcast) ⇒ 62 usable. $2^{h}$ saare patterns count karta hai, jinmein woh do reserved bhi shamil hain.

Trap 2: "Bada prefix /n = bada network." Yeh sahi kyun lagta hai: bada number bada lagta hai. Fix: Bada $n$ = zyada network bits = kam host bits = chhota network. /30 (2 hosts) bahut chhota hai; /8 bahut bada hai.

Trap 3: "Class C mein hamesha 254 hosts hote hain toh main ise chhota nahi bana sakta." Yeh sahi kyun lagta hai: classful soch. Fix: CIDR classless hai — tum ek /24 ko /30s mein subnet kar sakte ho (jaise point-to-point links /30 ya /31 use karte hain).

Trap 4: Kisi subnet ka network address choose karna jo block size ka multiple nahi hai. Fix: Network addresses hamesha us octet mein block size ke multiples hote hain (/26 ke liye 0, 64, 128, 192).

Recall Feynman: ek 12-saal ke bachche ko explain karo

Socho har computer ek ghar hai aur uska address ek lamba number hai. Number ka pehla hissa gali ka naam (network) hai aur aakhri hissa ghar ka number (host) hai. Postman (router) ko sirf galiyan jaanni chahiye, har ghar nahi — toh woh sirf aage ka hissa padhta hai. /24 ka matlab hai "pehle 24 digits gali hain, baaki ghar hain." Jab ek gali mein bahut zyada ghar ho jaate hain, toh hum use chhoti galiyon mein todh dete hain — kuch aur digits ko gali ke naam ka hissa maan kar — yahi subnetting hai. CIDR bas yeh allow karta hai ki gali/ghar ki line hum kahi bhi kaat sakein fixed jagahon ki jagah, taaki ghar ke numbers waste na hon.

IPv4 address mein kitne bits hote hain?

32 bits, 4 octets ke roop mein likha jaata hai (dotted decimal), har ek 0–255.

IPv4 addresses ki total sankhya?

2^{32} \approx 4.29

billion.

/n network mein usable hosts ka formula?

2^{32 - n} - 2

(network + broadcast subtract karo).

Host count mein se 2 kyun subtract karte hain?

All-zeros host = network address, all-ones host = broadcast; dono ko assign nahi kiya ja sakta.

Class A, B, C, D, E ke leading bits?

0, 10, 110, 1110, 1111.

Class C ka first-octet range?

192–223.

CIDR mein /n ka matlab kya hai?

n = network (mask) bits ki sankhya; pehle n bits network ke hain, baaki host ke.

/26 ka subnet mask?

255.255.255.192 (last octet 11000000 = 192).

s bits borrow karne par subnets ki sankhya?

2^{s}

h host bits ke saath hosts per subnet?

2^{h} - 2

/27 ke liye last octet mein block size?

2^{32 - 27} = 32

, toh boundaries 0,32,64,...

Kya bada prefix matlab bada network hota hai?

Nahi — bada /n = zyada network bits = kam host bits = chhota network.

127.0.0.0/8 range kis kaam aati hai?

Loopback (127.0.0.1 = localhost).

Teen private IPv4 ranges (RFC 1918)?

10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16.

CIDR aggregation/supernetting kya hai?

Contiguous networks jo prefix share karte hain unhe ek chhote-prefix route mein combine karna taaki routing tables chhoti ho sakein (jaise do /24s → ek /23).

10.20.30.45/27 ka network address?

10.20.30.32 (block size 32; 32 ≤ 45 < 64).

Connections

IPv6 — addressing and notation (128-bit successor jo IPv4 exhaustion solve karta hai)
Subnet Mask and ANDing (routers network part kaise extract karte hain)
NAT — Network Address Translation (private ranges Internet par kyun kaam karte hain)
Routing Tables and Longest Prefix Match (CIDR aggregation kyun matter karta hai)
DHCP and APIPA (link-local 169.254.0.0/16)
Binary and Positional Number Systems (octet conversion ki foundation)