44
votes
Why can't devices on different VLANs, but on the same subnet, communicate?
One of the things VLAN's do is take a physical switch and break them up into multiple smaller "virtual" switches.
Meaning this Physical depiction of One switch and Two VLANs:
Is identical in ...
- 15.1k
40
votes
Accepted
What is the difference between 0.0.0.0 and a loopback IP address?
The statement:
The IP address 0.0.0.0 [...] means ‘‘this network’’ or ‘‘this host.’’
is misleading. It is not a "or" but "This host on this network."
From RFC1122:
{ 0, 0 }
...
- 19.8k
40
votes
Accepted
Why is 10.1.255.255 an invalid broadcast address?
I believe the book wrongly assumes network classes are still in effect. So a) would be a "Class A" network, where 10.255.255.255 would be the broadcast address. Another hint: There is no explicit ...
- 8,127
30
votes
Why can't devices on different VLANs, but on the same subnet, communicate?
The whole point of Virtual LAN, is to create separate Layer 2 LANs on a single physical device.
It is like building an armored and sonic-proof wall in a room to create 2 rooms. The people in each ...
- 19.8k
28
votes
How do you calculate the prefix, network, subnet, and host numbers?
Part 1 of 2
IPv4 Math
With an IPv4 address and the network mask, the network mask length, or host mask, you can calculate the Network Address, Broadcast Address, Total Addresses, Usable Addresses, ...
- 100k
28
votes
What is the practical use of a /31 CIDR or 255.255.255.254?
A /31 network actually has two usable hosts for a point-to-point link. See the Standards Track RFC 3021, Using 31-Bit Prefixes on IPv4 Point-to-Point Links (published in December 2000):
Abstract
With ...
- 100k
22
votes
How do you calculate the prefix, network, subnet, and host numbers?
Continued from Part 1...
Part 2 of 2
Subnet IPv4 Networks
Subnetting a network is creating multiple, longer networks from a network address and mask. The basic idea is that you borrow high-order ...
- 100k
19
votes
Accepted
What are common sizes to split a /29 - /32 IPv6 subnet?
Some simple guidelines that work most of the time:
Dividing your /29
The standard size of your allocation from RIPE NCC is a /32
A /32 is a well-accepted prefix size in the global routing table
You ...
- 6,780
19
votes
Accepted
Why can't devices in different subnets talk with each other?
Devices in different subnets can communicate. That is the purpose of a router. Routers route packets between different networks.
Even if devices in different networks are on the same layer-2 ...
- 100k
19
votes
Accepted
What happens when a subnet reaches capacity?
A subnet (network) is really just a collection of contiguous addresses within a binary mask. It is simply a logical way to divide address block. If you run out of addresses in a network (subnet), then ...
- 100k
19
votes
Accepted
Is there anything stopping me from using Class A addresses on my own network?
As long as you are translating your "15.0.0.0" address space to something unique on the Internet that doesn't overlap, things will "work fine".
However, you won't be able to ...
- 15.1k
17
votes
How does a host in one subnet know to forward the packet to the router MAC, when trying to send a packet to another subnet
The sending device uses the subnet mask to determine if the remote host is in it's local network or not.
If the IP is within the subnet of the local machine, it uses ARP to determine the MAC address ...
- 2,269
16
votes
Accepted
What is IP squat space
IPv4 address space is in short supply, so some people decide to use IP space ( allocated, but not advertised) that doesn't belong to them. The consequences are pretty well described in the article ...
- 67.8k
14
votes
Accepted
Using IP subnet at multiple datacenters
Connect the two DCs with a private connection. Then advertise the /24 at both Data Centers.
When traffic arrives at one DC for the other, your internal devices route or switch the traffic as ...
- 737
13
votes
What is the difference between 0.0.0.0 and a loopback IP address?
If I am correct, a loopback IP address refers to the current host.
No. Traffic sent to a loopback address loops back inside the host. You can send traffic to a loopback address as the destination ...
- 100k
13
votes
What is IP squat space
What is IP squat space
Space that someone uses to number their networks even though it is either allocated to someone else or may be allocated to someone else in the future.
"squat space" is ...
- 13.6k
12
votes
Is there anything stopping me from using Class A addresses on my own network?
Any network addresses you use in your own company that are in use or assigned to a different company on the public Internet will be inaccessible to your users trying to reach those addresses on the ...
- 100k
11
votes
Why do I need to specify a subnet mask on interface?
Super simple answer...
It tells the computer what range of addresses it can talk to before sending the traffic to the gateway to decide where it goes next.
The traffic in that range never goes to ...
- 938
11
votes
Accepted
Why are IPv6 addresses so long?
By convention, each device on the LAN under IPv6 gets a /64 subnet,
right?
No. Each network is almost always a /64 network. Each host still gets one or more addresses on the network, but I don't ...
- 100k
11
votes
Accepted
Why is the IPv6 link-local address range fe80::/10 rather than fe80::/64?
Why isn't the link-local address format simply specified as FE80::/64?
Because that would leave the option open for using other, non link-local networks that start with fe80 as the first word. For ...
- 100k
11
votes
Accepted
Why couldn't a nework address be used as as broadcast address as well?
What is the point of reserving two IP addresses in a subnet when one might suffice?
[...] If it's theoretically possible, why is it avoided?
We live with our history. The distinction between address ...
- 16.3k
10
votes
Accepted
VRFs, VLANs and subnets: difference
Each fills a different purpose, and all three may be part of an overall solution. Let's start with the oldest concept first.
Subnets are the IP world's way of determining what devices are "...
- 13.6k
10
votes
Accepted
Finding Subnet Ranges of IPv6
Basically, with IPv6, most subnets will just be /64. Using subnets of other sizes, with a couple of exceptions, will break features of IPv6. See RFC 5375, IPv6 Unicast Address Assignment ...
- 100k
10
votes
Accepted
Why do I need to specify a subnet mask on interface?
The router uses the IP address and mask configured on an interface to determine what network is attached to that interface. That network and mask then become part of the routing table as a directly ...
- 100k
10
votes
Accepted
Calculate subnet mask from IP Address
How can we calculate the subnet mask (255.x.x.x) for this given IP ?
You can't. To find the prefix, you need the address and mask, or the address and prefix length.
See the answers to this question ...
- 100k
10
votes
BGP vs OSPF path selection with /24 vs /23`
The route with the longest mask (/24) is always preferred regardless of how it’s learned. The protocol doesn’t matter.
- 67.8k
10
votes
How will be the ARP Request (Unicast/Broadcast) if Subnet Mask is Unknown
A device NEVER knows the subnet mask of another device.
A device knows its IP address and its own subnet mask and based on this information knows in which network it resides.
When host A sends a ...
- 19.8k
10
votes
Does IPv6 have similar concept of network mask?
IPv6 uses network masks exactly the same way as IPv4. The only real difference is that they're much longer (128 bits). IPv6 uses the slash notation exclusively (no dotted decimal), so masks range ...
- 67.8k
9
votes
How do you calculate the prefix, network, subnet, and host numbers?
(In an attempt to keep all the netmask answers in one place, after the other excellent answers, I've added this one about a visual method.)
Subnet Sizing Based on Number of Hosts
This is for the ...
- 16.3k
9
votes
Accepted
What is the use of the IPv6 scope ID?
Given the IPv6 Link-Local addressing, where every interface has the same network (fe80::/10), there must be some way to distinguish which specific network is meant when referring to a link-local ...
- 100k
Only top scored, non community-wiki answers of a minimum length are eligible