101
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Why isn't 255.255.249.0 a valid subnet mask?
It's not a valid subnet mask because it doesn't define a prefix. If you convert it to binary, you'll see that its '1'-bits (and/or its '0'-bits) are not all contiguous. This is explicitly forbidden by ...
- 454k
78
votes
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Why are subnet-masks relevant for the individual computer on the network?
Your original assumptions are not entirely correct. What you call a "router" is two devices in one – a two-port router internally connected to a multiple-port Ethernet switch. (Here's an example ...
- 454k
59
votes
What does it mean to have a subnet mask /32?
/32 addressing
Generally speaking, /32 means that the network has only a single IPv4 address and all traffic will go directly between the device with that IPv4 address and the default gateway. The ...
- 4,658
37
votes
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What does it mean to have a subnet mask /32?
There's a bit of confusion here; that /32 doesn't refer to the size of any (sub)network, but to the range of addresses that particular routing table entry applies to. Usually the two are the same (...
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36
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My IP addresses changed from 192.168.2.xxx to 192.168.1.xxx. I can't reach the switch anymore
Temporarily change one of your computers' IP address to be in the 192.168.1.0/24 range and you should be able to connect to your switch (you will be disconnected from the Internet). Then just ...
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31
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Why are subnet-masks relevant for the individual computer on the network?
How does a computer know if a destination address is in the same subnet on in another?
Checking the local adddress and the subnet mask.
Let's check a couple examples:
If my computer has the IP 192....
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18
votes
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How to correctly set the subnet mask installing Linux Ubuntu 20.04 server? ERROR: '172.16.182.216' is not contained in '255.255.255.0/24'
It asks for the subnet specification in the 1st field, not the subnet mask.
The subnet is 172.16.182.0 and it uses a 24 bit (255.255.255.0) mask so the CIDR notation for the subnet is 172.16.182.0/24
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17
votes
Why isn't 255.255.249.0 a valid subnet mask?
The mask needs to contiguously have high order bits on to mask the network address under which the subnet addresses are. 249 does not have this property, as it is 11111001 in binary, there are zeroes ...
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11
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What does (Preferred), (Tentative) and (Duplicate) mean against an IPv4 Address with ipconfig
Preferred is what your client will ask the DHCP server for when it gets/renews the lease, but tentative is what it got in the response. Someone else probably has a lease on the preferred address. ...
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11
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Why are subnet-masks relevant for the individual computer on the network?
Something non-obvious about IP is that every IP device is itself a router.
This can be seen on a normal PC with the command "route print". You are connected to two networks: your local Ethernet or ...
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9
votes
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Is the following IP address mask legal?
It is not valid. A subnet mask needs to be continuously 1-filled from the left. This is the reason why you can use the alternative notation with /x, where x describes the number of 1-bits (the size of ...
- 1,268
8
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Non-contiguous Subnet Mask
CIDR (Classless Inter-Domain Routing) came along in 1993 in RFC 1519 (now RFC 4632 / BCP 122 ) and spelled the end for non-contiguous subnet masks:
"An implementation following these rules should ...
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8
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Why does it seem like I am losing IP addresses after subnetting with the subnet mask of 255.255.255.192/26?
Why it is not showing in subnets?
Because calculator.net is very much stuck in the obsolete "classful network" way of thinking – and even gets that wrong, too. Don't use it.
Note how it has ...
- 454k
7
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Why are subnet-masks relevant for the individual computer on the network?
I see this mentioned in some of the other answers here but I think it could be clearer: On computers with multiple network interfaces, the subnet mask may be used to automatically determine which ...
- 10.8k
7
votes
What does it mean to have a subnet mask /32?
It is just CIDR value. You can learn more in here for CIDR.
TL;DR
A CIDR network address looks like this under IPv4:
192.30.250.00/18
The "192.30.250.0" is the network address itself and ...
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7
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Why does it seem like I am losing IP addresses after subnetting with the subnet mask of 255.255.255.192/26?
Reminder: there is no "class B" anymore. CIDR appeared in 1993 — 30 years ago, you probably had never seen any classful networking in your career. Just stop using these terms, "class A&...
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6
votes
How does IPv4 subnetting work?
IP subnets exist to allow routers to choose appropriate destinations for packets. You can use IP subnets to break up larger networks for logical reasons (firewalling, etc), or physical need (smaller ...
6
votes
Why are subnet-masks relevant for the individual computer on the network?
TCP/IP could have been designed as you suggest -- leaf nodes would send everything to the router, and it would forward it to the target, which might be on the same subnet as the sender.
But this ...
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6
votes
Why aren't public IP addresses always displayed with their subnet mask?
All systems that use TCP/IP to talk to other systems consult a local "Fowarding Information Base" or local routing table which is maintained by the system.
A routing table entry basically looks like ...
- 74k
6
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Why subnet prefix is required when setting up a static address with nmcli in Linux?
First of all, NetworkManager's requirements are exactly the same as just about every other OS, ever. Windows requires you to enter a subnet mask; macOS requires you to enter a subnet mask; and ...
- 454k
6
votes
Device category between router and firewall (subnetting but nothing more)
Your best bet is to buy a cheap mini-PC and run pfSense.
pfSense is the software part of a very, very high level Firewall/Router and runs on commodity hardware (x64 PC). It is used heavily in ...
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5
votes
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Same IP range, Different Subnet range
Yes, they should be able to ping each other, in both directions: the reason is that the network of PC-B is wholly contained inside the network of PC-A, and PC-A is located in that portion of its ...
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5
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Why are subnet-masks relevant for the individual computer on the network?
If we look at a routing table (this happens to be my desktop machine):
ip route
default via 172.20.25.1 dev eth1
172.20.25.0/24 dev eth1 proto kernel scope link src 172.20.25.33
192.168.0.0/24 dev ...
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5
votes
Accepted
which subnet i can use for 51. and 62
These are public IP addresses, and thus you should not be playing with them. Take a step back, think about what you're trying to achieve and then update your question with more details.
In addition ...
- 20k
5
votes
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Why don't DNS queries return the subnet mask?
The short answer is DNS does not have anything to do with configuring routing, there are two completely different things.
Think of your routing table as a set of rules that determine how IP packets ...
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5
votes
Why aren't public IP addresses always displayed with their subnet mask?
The subnet mask is useful for the host itself and to the routing equipment.
Your PC doesn't need to know the subnet mask of a remote device, it only needs to know the address and comparing with its ...
- 17.5k
5
votes
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Semantics of how subnets work
"The router will forward a packet sent by host1 down its default gateway if and only if for all it's connected interfaces, the interface's configured address bitwise AND with host1's subnet mask does ...
- 454k
5
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What does it mean to have a subnet mask /32?
easiest thing is web search and read articles related to subnet mask and subnet mask binary shorthand and CIDR
and also check out subnet calculators
the /32 is the CIDR (shorthand) and refers to how ...
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5
votes
Why isn't 255.255.249.0 a valid subnet mask?
Formally speaking, it actually is a valid subnet mask. As the name implies, a subnet mask is simply a bitmask pattern (any pattern) that tells you which portions of the address define the network as a ...
- 151
5
votes
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Subnet cross-access with three routers
By setting the subnet masks of the two LAN routers to /23, I thought I could enable access to the respective subnets, but to no avail.
No, that's very nearly the opposite of how it works.
The subnet ...
- 454k
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