IPv4 subnetting
- Hosts per subnet
- 2(32 − prefix) − 2 (/31: 2, /32: 1)
- Block size
- 256 − (mask octet)
- Network address
- IP AND mask
- Broadcast address
- IP OR (NOT mask): all host bits set to 1
- Subnets from n borrowed bits
- 2n
Computer Networks: cram pack
Formula sheets, the key method steps for every topic and 6 practice problems, with full solutions at the back.
1Formulas and methods
Number systems
- Place values
- binary 2k, octal 8k, hex 16k
- To decimal
- Σ dk·bk
- From decimal
- divide by the base, read the remainders bottom-up
- Hex ↔ binary
- one hex digit = 4 bits
- Two’s complement
- invert the bits and add 1
2Practice problems
1.IPv4 subnettingMedium
The network 192.168.10.0/24 must be divided into at least 10 equal subnets. If you borrow as few bits as possible, how many usable hosts H does each subnet have?
192.168.10.0/2410 subnets
H =
2.IPv4 subnettingHard
A host has the address 10.20.252.86/29. What is the fourth octet x of the last usable host address in its subnet?
10.20.252.86/29
x =
3.Computer networksMedium
Why is UDP often preferred for live video calls?
- AUDP encrypts data automatically
- BUDP is required for all video
- CUDP guarantees every packet arrives, so the picture never freezes
- DLow delay matters more than retransmitting every lost packet
4.Computer networksHard
A student runs a game server on a PC behind a home NAT router. Friends on the internet cannot connect. What is the usual fix?
- ATurn off the PC's DNS client
- BGive the PC a different MAC address so the router can find it from outside
- CConfigure port forwarding on the router to the PC's private address
- DChange the PC's subnet mask to /32
5.Number systemsMedium
Set the bits so they store −100 in 8-bit two's complement. Read as an unsigned number, what is the pattern's value U?
N = −1008 bit
U =
6.Number systemsHard
Set the bits to the 8-bit result of 1000 0010 << 1. What is the result's value R in decimal?
1000 0010<< 1
R =
3Solutions
1.IPv4 subnetting
Answer: H = 14
IPv4 subnetting: an address has 32 bits. The prefix /p says how many leading bits identify the network; the remaining 32 − p bits number the hosts. A subnet has 2³²⁻ᵖ addresses, but the first (network address) and last (broadcast address) cannot be given to hosts, so 2³²⁻ᵖ − 2 hosts fit. Exceptions: a /31 (point-to-point link, RFC 3021) uses both of its 2 addresses for hosts, and a /32 is one single address (a single host or a loopback).
Bits to borrow
24 = 16 ≥ 10 → b = 4, /28
Hosts per subnet
24 − 2 = 14
Answer
H = 14
2.IPv4 subnetting
Answer: x = 86
IPv4 subnetting: an address has 32 bits. The prefix /p says how many leading bits identify the network; the remaining 32 − p bits number the hosts. A subnet has 2³²⁻ᵖ addresses, but the first (network address) and last (broadcast address) cannot be given to hosts, so 2³²⁻ᵖ − 2 hosts fit. Exceptions: a /31 (point-to-point link, RFC 3021) uses both of its 2 addresses for hosts, and a /32 is one single address (a single host or a loopback).
Block size
23 = 8
Subnet of this host
network: ⌊86/8⌋·8 = 80
broadcast: 80 + 8 − 1 = 87
usable hosts: 81 … 86
Answer
x = 86
3.Computer networks
Answer: D · Low delay matters more than retransmitting every lost packet
Why this is the answer
Retransmitting lost packets would add delay; real-time media tolerates some loss, so UDP-based protocols like RTP are used.
Answer
D · Low delay matters more than retransmitting every lost packet
4.Computer networks
Answer: C · Configure port forwarding on the router to the PC's private address
Why this is the answer
NAT creates mappings only for connections started from inside. A port-forwarding rule maps a public port to the internal host so inbound connections can reach it.
Answer
C · Configure port forwarding on the router to the PC's private address
5.Number systems
Answer: U = 156
Setting bits: each bit is worth a power of two (128, 64, …, 1). One hex digit is exactly four bits, and in 8-bit two's complement the leftmost bit is worth −128. Bitwise operations work on each bit position separately.
Invert and add one
100 = 0110 0100
inverted: 1001 1011
+1 → 1001 1100
Read it as unsigned
1001 1100₂ = 256 − 100 = 156
Answer
U = 156
6.Number systems
Answer: R = 4
Setting bits: each bit is worth a power of two (128, 64, …, 1). One hex digit is exactly four bits, and in 8-bit two's complement the leftmost bit is worth −128. Bitwise operations work on each bit position separately.
Bit by bit
Shifting left by 1 moves every bit 1 places to the left; bits that fall off the left end are lost and zeros come in on the right.
1000 0010 << 1
= 0000 0100
Value of the result
0000 0100₂ = 4
Answer
R = 4
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