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215

A simple explanation: The /<number> is how a computer can quickly calculate what is part of its network and what is not. It represents the bit length of the subnet mask, as indicated above. The subnet mask is like masking when painting. You place a mask over what you DO NOT want to paint on. The subnet mask is a way to calculate the network ...


179

Calculating the Netmask Length (also called a prefix): Convert the dotted-decimal representation of the netmask to binary. Then, count the number of contiguous 1 bits, starting at the most significant bit in the first octet (i.e. the left-hand-side of the binary number). 255.255.248.0 in binary: 11111111 11111111 11111000 00000000 ...


173

The slash and the number following it is just a shorthand way to write a subnet mask. It's called CIDR (Classless Inter-Domain Routing) notation. It's also commonly referred to as the prefix length. The number after the slash represents the number of consecutive 1's in the subnet mask. For example, 192.168.10.0/24 is equal to the network 192.168.10.0 with a ...


44

If they are used interchangeably then they are used incorrectly. Subnet refers to particular IP network, such as 192.0.2.0/28. VLAN refers to 802.1Q standard, in which you can essentially give each port unique MAC address table, effectively separating them from each other. VLAN may transport one or more subnet (but does not have to, it may be ...


40

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 network size specified (/24, /27, ..) so it is implied you know about network classes. Classical example of outdated literature.


39

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 operation to this Logical depiction of the same topology: Even if the IP addresses in the 2nd image were in the same Subnet, you'll notice there is no "link" between ...


38

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 } This host on this network. MUST NOT be sent, except as a source address as part of an initialization procedure by which the host learns ...


33

It's likely that the subnet masks are throwing you off. As long as you keep in mind that the below rules no longer apply, you should be fine. Ultimately classful addressing came down to the most significant (or "leading") bits in the address. Nothing more, nothing less. Class A: Most significant bits starts with 0 Class B: Most significant bits start with ...


29

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 half of the room can no longer communicate with the people in the other half of the former room. So you have two hosts on two distinct L2 networks without ...


28

If you are using /28 network mask then 148.26.1.176 is the network address and therefore is not an assignable IP address for this subnet. For subnet 148.26.1.176/28 the range of assignable IP address is 148.26.1.177 to 148.26.1.190, as 148.26.1.176 is the network address and 148.26.1.191 is the broadcast address


28

The slash following the IP address is the abbreviation for the subnet mask. The binary version of a subnet mask is going to be comprised of ones and zeros just as the binary verison of an IP address would be, however, the ones in a subnet mask are all consecutive. The amount of ones in the subnet mask is equal to the number of the abbreviation. For ...


26

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 ever-increasing pressure to conserve IP address space on the Internet, it makes sense to consider where relatively minor changes can be made to ...


19

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: If a suitable subnetting scheme can be designed and is supported by the equipment concerned, it is advisable to use the 24-bit block (class A network) of ...


19

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 can get a /29 just by asking for it Conclusion: Get a /29 and start using the a /32, save the other /32s for when you deploy to other countries, continents etc. ...


19

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 you are simply out of addresses in that network. Adding any more hosts would require reclaiming unused addresses, expanding the network (may not be possible), ...


18

While the idea behind classful addressing is now obsolete as classless interdomain routing (CIDR) has been in use for decades(the original RFC1519 was published in 1993), your first answer is the historically correct one. The second set of networks you list are from RFC1918, and define private use address ranges. There is a single /8 network within the ...


18

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 interested in other examples, I wrote a few blog posts on the subject - http://www.oznetnerd.com/category/subnetting/ Admins, if this post is considered spam, ...


17

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 communicate (easily) to any users who own "the real" 15.0.0.0/8. At the moment some of that space seems to be owned by HP: $ whois 15.0.0.1 [Querying whois.arin.net] [whois.arin.net] ...


17

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 of the remote host. If it's outside, it queries it's local routing table to find the next hop of that IP, and sends out an ARP query to find the MAC address ...


16

There's no solution which satisfies all requirements, your answer of making 8 /27's is probably the most logical one. You can easily verify this by trying to divide 256 by 7, you can't do this without a remainder. Also, all subnets need to have a size which is a power of 2, and 7 * 32 (a /27) = 252, 8 * 16 (a /26) = 512, so there's no way to do this without ...


15

I do not want to take anything away from Mike Pennington's excellent answer, which I have relentlessly promoted, but I keep seeing questions that are not directly addressed by his answer, and I have created something that was originally based on Mike's answer, but I have more information to address questions that have popped up over time. Unfortunately, it ...


14

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 required via the private link. Second option depends on available connectivity options, you might be able to acquire a Layer2 (switched) link between both DCs and ...


14

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 you quote.


13

Subnets (L3) and VLANs (L2) are on different layers. The terms should not be used interchangeably. A VLAN can contain one or more L3 prefixes ("Subnets"). For a layman this could cause confusion. Often people don't understand that these two are connected but not the same. People might say The host located in our server subnet or The server located in the DMZ ...


13

Continued from the previous answer... Part 2 of 2 Selecting an IPv4 Network Gateway (Router) Address A gateway is a host on the network that knows how to forward packets to other networks, and it can be assigned any usable network host address. Some people just randomly assign gateway addresses to any usable network host address, some people always assign ...


13

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 that MAC address (end of story). If the host needs to send an ARP request to resolve the layer-3 IP address to a layer-2 MAC address, this is where it gets tricky....


12

This is a very informal way of looking at the difference between VLANs and subnets, but it's not inaccurate (just incomplete). It may help networking newcomers get a little bit closer to the right mental picture. Two different VLANs on a single switch or host are like two physically separate switches. They partition the MAC address space, in that ...


12

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. A more appropriate way to do things would be to set your device IP to 10.0.0.2, your gateway to 10.0.0.1, and your subnet mask to 255.255.255.252.


12

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 address and then read it. This is useful in testing. What is the difference between 0.0.0.0 and a loopback IP address then? Addresses in the 0.0.0.0/8 network ...


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