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42 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 ...
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40 votes
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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 } ...
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40 votes
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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 ...
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29 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 ...
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27 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 ...
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21 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, ...
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20 votes

How do you calculate the prefix, network, subnet, and host numbers?

The answer above hits the nail on the head perfectly. However, when I first started out, it took me a few different examples from a couple of sources for it to really hit home. Therefore, if you're ...
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19 votes
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Why are there 3 ranges of private IPv4 addresses?

Back when the RFC for private addressing was proposed, classful addressing was still common. The reasons for the three address ranges are found in RFC 1918, Address Allocation for Private Internets: ...
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19 votes
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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 ...
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18 votes
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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 ...
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18 votes
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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 ...
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17 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 ...
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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 ...
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  • 2,229
15 votes
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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 ...
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14 votes

Setting subnet mask to 255.255.255.255

Yes, there's a problem. Your default gateway needs to be in the same subnet as your device. By setting the subnet mask to 255.255.255.255, you've told the computer that nothing else is in its subnet....
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14 votes
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What would happen to Ping if two PCs have same IP address with same Subnet Mask on same network?

Actually, the ping gets sent to a layer-2 address if they are on the same LAN. Assuming ethernet, the sending host may have a MAC address in its ARP cache, and the pings gets sent to the host with ...
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14 votes
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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 ...
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  • 655
13 votes

How to divide network into subnets?

Remember that everything is binary, so a power of 2. Addresses and masks have 32 bits. You are given 8 bits (2^8 = 256) to work with since your network is /24 (32 - 24 = 8). The required number of ...
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  • 93.5k
12 votes
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Why do I get "% Warning: use /31 mask on non point-to-point interface cautiously" when I use a /31 subnet mask?

Your problem is that you are addressing in two separate networks: 216.21.5.0/31 and 216.21.5.2/31. The addresses you are using are in the same /30 network, but two separate /31 networks. You need to ...
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  • 93.5k
12 votes
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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 ...
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  • 93.5k
12 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 ...
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11 votes
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What is the purpose of network address?

They are functionally identical -- all-hosts subnet broadcast. In the early days, long long ago, the all-zero's address ("Network") was used for broadcast traffic. That was later changed to the all-...
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11 votes
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What is the correct mask to use with 224.0.0.0?

You can't use any addresses from 224.0.0.0 (except for multicast) through 255.255.255.254, and 255.255.255.255 is a Limited Broadcast address. Multicast addresses are 224.0.0.0/4. Masks are not used ...
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11 votes
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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 ...
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11 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 ...
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  • 93.5k
11 votes
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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 ...
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  • 15.9k
11 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 ...
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  • 11.6k
10 votes

What is the purpose of network address?

The use of "0" in a IP addresss was defined in RFC923 and carried over in successive RFCs: Special Addresses: In certain contexts, it is useful to have fixed addresses with functional ...
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  • 26.7k
10 votes
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What is the purpose of subnetting, if we have NAT?

NAT and subnetting solve two different problems. Problem 1: Previously IP addresses were divided into classes A, B and C. A class C address had a default subnet mask of 255.255.255.0 meaning 24 bits ...
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  • 606
10 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 ...
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