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
A subnet uses the mask 255.255.255.240. How many usable host addresses H does it have?
255.255.255.240
H =
2.IPv4 subnettingHard
A host has the address 172.20.230.78/18. What is the third octet x of its subnet's broadcast address?
172.20.230.78/18
x =
3.Computer networksMedium
What is the purpose of the TCP three-way handshake?
- ATo close a connection quickly
- BTo find the IP address of a host before data can be sent to it
- CTo establish a connection and synchronise sequence numbers
- DTo encrypt the data
4.Computer networksHard
In a TCP connection, what does the receive window (rwnd) control?
- AThe size of each IP packet header
- BHow much unacknowledged data the sender may send, based on the receiver's buffer
- CThe retransmission time-out, calculated from the measured round-trip time of each segment
- DThe number of open ports
5.Number systemsMedium
Set the bits so they store −64 in 8-bit two's complement. Read as an unsigned number, what is the pattern's value U?
N = −648 bit
U =
6.Number systemsHard
Set the bits to the 8-bit result of 0010 1110 >> 1. What is the result's value R in decimal?
0010 1110>> 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).
Prefix length
255.255.255.240 = /28
Usable hosts
24 − 2 = 14
Answer
H = 14
2.IPv4 subnetting
Answer: x = 255
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 in the third octet
26 = 64
Subnet of this host
network: 172.20.192.0
broadcast: 172.20.255.255
Answer
x = 255
3.Computer networks
Answer: C · To establish a connection and synchronise sequence numbers
Why this is the answer
The handshake sets up a connection and agrees initial sequence numbers on both sides before data is sent.
Answer
C · To establish a connection and synchronise sequence numbers
4.Computer networks
Answer: B · How much unacknowledged data the sender may send, based on the receiver's buffer
Why this is the answer
The receiver advertises how much buffer space it has; the sender limits unacknowledged data accordingly, preventing receiver overload.
Answer
B · How much unacknowledged data the sender may send, based on the receiver's buffer
5.Number systems
Answer: U = 192
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
64 = 0100 0000
inverted: 1011 1111
+1 → 1100 0000
Read it as unsigned
1100 0000₂ = 256 − 64 = 192
Answer
U = 192
6.Number systems
Answer: R = 23
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 right by 1 moves every bit 1 places to the right; bits that fall off the right end are lost and zeros come in on the left.
0010 1110 >> 1
= 0001 0111
Value of the result
0001 0111₂ = 23
Answer
R = 23
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