Personal Blogs
I'm officially on holiday for 2 weeks from tomorrow — finally it gives me the chance to be relaxed, focused and think about all the great material I've overlooked and rushed through.
This is probably an obvious point to many people — it's only now I'm beginning to realise why a lot of study advice keeps reiterating —take lots of short breaks, don't try to process too much information in one go.
The thoughts need to absorb your subconscious cognitive processes for one to truly understand concepts.
Why are classful subnet masks of a fixed length wasteful?
Because when you want to subnet your network,
you are constrained by doing it in powers of 2.
That is either 2,4,8,16,32,64,128 etc.
This is one of the reasons for changing to a classless addressing scheme, this means your subnet mask can be a variable length.
Which means your can subnet your network precisely to the required number of required subnetworks.
I just typed ipconfig /all into the cmd.
The following was returned
Connection-specific DNS Suffix . : lan
Description . . . . . . . . . . . : Broadcom 802.11n Network Adapter
Physical Address. . . . . . . . . : 74-E5-43-7D-8B-65
DHCP Enabled. . . . . . . . . . . : Yes
Autoconfiguration Enabled . . . . : Yes
Link-local IPv6 Address . . . . . : fe80::39d0:ff36:ff66:2215%15(Preferred)
IPv4 Address. . . . . . . . . . . : 192.168.1.78(Preferred)
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Lease Obtained. . . . . . . . . . : 26 January 2020 07:13:40
Lease Expires . . . . . . . . . . : 27 January 2020 07:13:40
Default Gateway . . . . . . . . . : 192.168.1.254
DHCP Server . . . . . . . . . . . : 192.168.1.254
DHCPv6 IAID . . . . . . . . . . . : 259319107
DHCPv6 Client DUID. . . . . . . . : 00-01-00-01-25-1D-01-86-D4-BE-D9-53-C6-37
DNS Servers . . . . . . . . . . . : 192.168.1.254
I know it's beyond the scope of this module, that is subnetting;
My IPv4 Address is 192.168.1.78
I have a subnet mask of 255.255.255.0 --> this is a network, network, network. host pattern
which means I'm on the 192.168.1.0 network.
now if I had a subnet mask of 255.255.255.240 what do you think this means?
Answer:
I think it means I've sub-netted my default subnet mask, I've taken or stolen binary bit's from the octet host portion of my IPv4 address.
This means more networks but at a cost of less hosts.
This means in effect we are creating smaller subnetworks out of the larger one, dividing it up .
But realising that although you will increase the amount of networks, each range will hold a smaller amount of hosts per subnetwork.
how many bits did I steal to do this?
Answer:
convert the custom subnet dotted decimal host portion to binary,
128 64 32 16 8 4 2 1 - binary values working
1 1 1 1 0 0 0 0 240 - 128 = 112
112 - 64 = 48
48 - 32 = 16
16 - 16 = 0
check: 128 + 64 + 32 + 16 = 240
111100002 = 24010 (denary value)
This means 4 bits where taken from the host octet portion of the IPv4 address.
next question :
How many usable new subnetworks does this give me?
to work this out :
use formula : Number of subnets = 2s
so 24 = 16 so 16 subnets.
How many hosts per subnet ?
use formula : Number of hosts per subnet = 2h -2
I originally had 8 bits in the host portion and 8 - 4 = 4
so 24 - 2 = 14
Total number of subnets (subnetworks) = 16
Total number of host address per subnetwork = 14
workout the difference in binary between the default network address binary number and the
custom subnetwork binary number: ( Note the count starts at 0 !)
Address Ranges per subnetwork
(0) 192.168.1. 0 0 0 0 . 0 0 0 0 192.168.1.0 - 192.168.1.15
(1) 192.168.1. 0 0 0 1 . 0 0 0 0 192.168.1.16 - 192.168.1.31
(2) 192.168.1. 0 0 1 0 . 0 0 0 0 192.168.1.32 - 192.168.1.47
(3) 192.168.1. 0 0 1 1 . 0 0 0 0 192.168.1.48 - 192.168.1.63
(4) 192.168.1. 0 1 0 0 . 0 0 0 0 192.168.1.64 192.168.1.79
(5) 192.168.1. 0 1 0 1 . 0 0 0 0 192.168.1.80 - 192.168.1.95
(6) 192.168.1. 0 1 1 0 . 0 0 0 0 192.168.1.96 - 192.168.1.111
(7) 192.168.1. 0 1 1 1 . 0 0 0 0 192.168.1.112 - 192.168.1.127
(8) 192.168.1. 1 0 0 0 . 0 0 0 0 192.168.1.128 - 192.168.1.143
(9) 192.168.1. 1 0 0 1 . 0 0 0 0 192.168.1.144 - 192.168.1.159
(10) 192.168.1. 1 0 1 0 . 0 0 0 0 192.168.1.160 - 192.168.1.175
(11) 192.168.1. 1 0 1 1 . 0 0 0 0 192.168.1.176 - 192.168.1.191
(12) 192.168.1 1 1 0 0 . 0 0 0 0 192.168.1.192 - 192.168.1.207
(13) 192.168.1 1 1 0 1 . 0 0 0 0 192.168.1.208 - 192.168.1.223
(14) 192.168.1 1 1 1 0 . 0 0 0 0 192.168.1.224 - 192.168.1.239
(15) 192.168.1 1 1 1 1 . 0 0 0 0 192.168.1.240 - 192.168.1.255
So to conclude there are a total of 16 subnets or 16 subnetworks, that counting from 0 to 15 which is like the range function in python index starting at 0 !
Within each subnetwork division there are 14 usable host addresses
this is because each sub-network's starting address
per pool of IPv4 addresses is the address of that subnetworks address itself ! ( the lowest IPv4 address)
and the highest value address per pool of subnet-work addresses is that subnetwork's broadcast address.
super-netting is the opposite more hosts but at cost of less networks.
- Numeric problems -> the output is directly determined by the input.
Analyse multiple casesMultiple cases
Patterns Problem
TypePattern
UsePattern
NumberProblem
ClassNumeric Formula 4.1 Numeric Case
analysis (two cases)4.2 Numeric 4.3 Numeric Nested cases 4.4 Numeric Generate a sequence 4.5 Numeric Generate a list 2.1 Numeric Append to a list 2.2 Numeric Filter list 2.3 Numeric List Tranformation 2.4 Numeric Reduce 2.5 Counting
problemsNumeric Aggregate 2.6 Counting
problemsNumeric Search 2.7 Find a value Numeric Search 2.8 Find best value
- Calculate
- Formula problems
- Word problems
- Formula problems
- Case analysis
- Two Cases
- Multiple Cases
- Nested Cases -> efficient
- Generate a sequence
- Initialise and generate
- Terminatation condition - > Give a suitable condition to stop the generation
- Non-termination -> Under which conditions will the generation never stop ?
- Infinite loops
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