1.7 IPv4 Subnet Masks Flashcards
N10-009 Obj. 1.7 Given a scenario, use appropriate IPv4 network addressing. (13 cards)
What is Classless Interdomain Routing (CIDR)?
CIDR is a method of subnetting where subnet masks are represented by the number of bits (e.g., /24) instead of class-based defaults.
Transcript: “Instead, we use classless subnetting… referred to as Classless Interdomain Routing, or CIDR.” (0:15–0:23)
How is a /24 subnet mask represented in decimal notation?
/24 is the same as 255.255.255.0.
Transcript: “A /24 mask would be the same as 255.255.255.0.” (1:11–1:18)
When configuring a device, how might subnet mask input formats differ?
Operating systems usually expect decimal notation, while routers and switches may prefer CIDR.
Transcript: “In Windows… decimal version… router or switch… input /8.” (1:34–1:55)
What is the binary structure of a subnet mask?
A subnet mask has a contiguous series of 1’s (network) followed by 0’s (host).
Transcript: “The mask consists of a contiguous series of 1’s followed by 0’s.” (2:05–2:12)
How do you calculate the CIDR block from a binary subnet mask?
Count the number of 1’s in the binary mask
Transcript: “You would count how many 1’s… the subnet mask.” (2:44–2:50)
What does a /16 subnet mask signify?
It means the first 16 bits are for the network and the remaining 16 for hosts.
11111111.11111111.00000000.00000000
Transcript: “CIDR block notation would be /16… 16 bits are associated with the network.” (3:36–3:46)
How many network and host bits does a /26 mask have?
26 network bits, 6 host bits.
11111111.11111111.11111111.11000000
Transcript: “The /26 means that the first 26 bits… last 6 bits… host ID.” (4:20–4:26)
What is the CIDR and decimal notation for a mask with 12 network bits?
/12 in CIDR, 255.240.0.0 in decimal.
11111111.11110000.00000000.00000000
Transcript: “So a /12 CIDR notation is the same thing as 255.240.0.0” (6:15–6:22)
What is the decimal notation for a /19 subnet mask?
255.255.224.0
“They all add up to 19… 255.255.224.0” (7:06–7:13)
What is the decimal representation of a /26 subnet mask?
255.255.255.192
“/26… same as 255.255.255.192” (8:01–8:10)
What is the decimal notation for a /20 subnet mask?
255.255.240.0
“Decimal version of that binary subnet mask will be 255.255.240.0” (8:32–8:38)
What part of the address is defined by a /20 subnet mask?
20 bits for the network and 12 bits for the host.
“Network part… 20 bits… 12 bits available for host” (8:42–8:48)
Convert the following binary subnet masks to decimal:
00000000
10000000
11000000
11100000
11110000
11111000
11111100
11111110
11111111
00000000 = 0
10000000 = 128
11000000 = 192
11100000 = 224
11110000 = 240
11111000 = 248
11111100 = 252
11111110 = 254
11111111 = 255
Memorize this!
