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 10.20.114.41/28. What is the fourth octet x of the last usable host address in its subnet?
10.20.114.41/28
x =
3.Computer networksMedium
How does TLS typically use asymmetric and symmetric cryptography?
- AOnly asymmetric cryptography is used throughout
- BSymmetric methods agree keys and authenticate; asymmetric ciphers encrypt all the data
- COnly symmetric cryptography is used, with a fixed shared password
- DAsymmetric methods agree keys and authenticate; symmetric ciphers encrypt the data
4.Computer networksHard
DNS queries normally use UDP. When does DNS typically use TCP instead?
- AOnly when the domain name is longer than ten characters
- BFor responses too large for UDP and for zone transfers
- CFor every query sent from a web browser
- DOnly for reverse lookups of IP addresses
5.Number systemsMedium
Set the bits so they store −46 in 8-bit two's complement. Read as an unsigned number, what is the pattern's value U?
N = −468 bit
U =
6.Number systemsHard
Set the bits to the 8-bit result of 1011 0110 >> 2. What is the result's value R in decimal?
1011 0110>> 2
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 = 46
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
24 = 16
Subnet of this host
network: ⌊41/16⌋·16 = 32
broadcast: 32 + 16 − 1 = 47
usable hosts: 33 … 46
Answer
x = 46
3.Computer networks
Answer: D · Asymmetric methods agree keys and authenticate; symmetric ciphers encrypt the data
Why this is the answer
Public-key methods and certificates set up a shared secret and prove identity. Fast symmetric ciphers such as AES then protect the bulk data.
Answer
D · Asymmetric methods agree keys and authenticate; symmetric ciphers encrypt the data
4.Computer networks
Answer: B · For responses too large for UDP and for zone transfers
Why this is the answer
If a UDP response is truncated, the resolver retries over TCP. Zone transfers between name servers also use TCP because they carry lots of data.
Answer
B · For responses too large for UDP and for zone transfers
5.Number systems
Answer: U = 210
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
46 = 0010 1110
inverted: 1101 0001
+1 → 1101 0010
Read it as unsigned
1101 0010₂ = 256 − 46 = 210
Answer
U = 210
6.Number systems
Answer: R = 45
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 2 moves every bit 2 places to the right; bits that fall off the right end are lost and zeros come in on the left.
1011 0110 >> 2
= 0010 1101
Value of the result
0010 1101₂ = 45
Answer
R = 45
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