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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 ...
Eddie's user avatar
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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 } ...
JFL's user avatar
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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 ...
Sebastian Wiesinger's user avatar
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 ...
JFL's user avatar
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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, ...
Ron Maupin's user avatar
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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 ...
Ron Maupin's user avatar
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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 ...
Ron Maupin's user avatar
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19 votes
Accepted

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: ...
Ron Maupin's user avatar
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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 ...
Sander Steffann's user avatar
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 ...
Ron Maupin's user avatar
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19 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 ...
Ron Maupin's user avatar
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19 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 ...
Eddie's user avatar
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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 ...
Stuggi's user avatar
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16 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 ...
Ron Trunk's user avatar
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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 ...
Ron Maupin's user avatar
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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 ...
Criggie's user avatar
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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 ...
Ron Maupin's user avatar
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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 ...
Peter Green's user avatar
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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 ...
Ron Maupin's user avatar
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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 ...
Ron Maupin's user avatar
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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 ...
Fixitrod's user avatar
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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 ...
Ron Maupin's user avatar
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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 ...
jonathanjo's user avatar
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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 ...
Jimmy's user avatar
  • 606
10 votes
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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 "...
Peter Green's user avatar
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10 votes
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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 ...
Ron Maupin's user avatar
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10 votes
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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 ...
Ron Maupin's user avatar
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10 votes
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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 ...
Ron Maupin's user avatar
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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.
Ron Trunk's user avatar
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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 ...
JFL's user avatar
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