Questions tagged [ethernet]

Ethernet is the most common set of standards for wired connections in local area networks.

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How do VLANs work?

What are VLANs? What problems do they solve? I'm helping a friend learn basic networking, as he's just become the sole sysadmin at a small company. I've been pointing him at various questions/answers ...
Murali Suriar's user avatar
90 votes
17 answers
72k views

Should network hardware be set to "autonegotiate" speeds or fixed speeds?

We recently had a little problem with networking where multiple servers would intermittently lose network connectivity in a fairly painful-to-resolve way (required hard reboot). This has been going on ...
Jeff Atwood's user avatar
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82 votes
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How to check the physical status of an ethernet port in Linux?

I want to check if a specified ethX is physically up or down. How do I do that with the command line?
Jader Dias's user avatar
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65 votes
6 answers
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Why do ethernet cables have 8 wires?

This might seem a stupid question but why do Ethernet cables have 8 wires? Cat5 cables were just using 4 of the 8 wires, so only 4 are actualy 'needed'. Why not 12 or 16 wires?
ProgrammerAtWork's user avatar
54 votes
6 answers
93k views

e1000e Reset adapter unexpectedly / Detected Hardware Unit Hang

I have a Dell 1U Server with Intel(R) Xeon(R) CPU L5420 @ 2.50GHz, 8 cores running Ubuntu Server Kernel Version 3.13.0-32-generic on x86_64. It has dual 1000baseT networking cards. I have it set up to ...
Kyle Coots's user avatar
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24 votes
4 answers
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What is the "in-the-wire" size of a ethernet frame? 1518 or 1542?

According to the table here, it says that MTU = 1500 bytes and that the payload part is 1500 - 42 bytes or 1458 bytes (<- this is actually wrong!). Now on top of that you have to add IPv4 and UDP ...
chrisapotek's user avatar
22 votes
2 answers
20k views

What is the clock frequency inside 10Gb and 100Gb Ethernet cards?

As I understand, a 10Gb Ethernet card is capable of putting 10Gb every second on (say) a fibre optics cable. Now naively, for this to happen in hardware, one will need a 10GHz clock running the ...
Randomblue's user avatar
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22 votes
3 answers
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What is this wiring panel and what are these Ethernet "ports?"

In my office's building there is a stone-age LAN rack with some Ethernet ports I've never seen before. I need to find the name of this ports, if they have one, and then buy some cables or adapters. ...
Alessandro Cappello's user avatar
20 votes
2 answers
2k views

What's the Wireshark packet receiving and processing procedure on a Windows machine?

I am about to use Wireshark for some traffic monitoring on my Windows computer. While working on it, I was wondering how Wireshark manages to catch low level network packets before Windows does. ...
Hansi's user avatar
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3 answers
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Best way to get the MAC of eth0?

Is there a more efficient way to retrieve the MAC address of a NIC in Linux? This works: ip link show dev eth0 | awk ' /link\/ether/ { print $2 }' but can it be found via something like: cat /sys/...
brodul's user avatar
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20 votes
5 answers
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ARP broadcast flooding network and high cpu usage

Hoping someone here might have some insight to the issue we are facing. Currently we have Cisco TAC looking at the case but they are struggling to find the root cause. Although the title mentions ARP ...
Cold T's user avatar
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18 votes
4 answers
2k views

What can cause network packets consisting of PUU?

We have a system which is suffering from comms outages on a gigabit ethernet network. The traffic load on the network is such as to slightly stress a 100Mb network, but there are gigabit switches and ...
AlastairG's user avatar
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18 votes
4 answers
16k views

How does Ethernet know how long a frame is?

Looking at the Ethernet entry on Wikipedia, I can't figure out how it's indicated how long the Ethernet frame is. The EtherType/Length header field apparently can indicate either a frame type or an ...
dirtside's user avatar
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17 votes
3 answers
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Is one 10 gig port the same as ten 1 gig ports

I've been asking me this for a couple of days and after a bunch of searching I wasn't able to come up with a comprehensible answer, not even a theoretical one that makes sense in my head. I'm playing ...
Constantin Jacob's user avatar
17 votes
8 answers
53k views

What's the difference between a bridge and a switch?

I can't find any authoritative source that can concisely explain the difference between a bridge and a switch. As far as I can tell, most devices commonly referred to as "switches" fit the description ...
Dan Moulding's user avatar
17 votes
6 answers
108k views

How to test cat5/cat6 cable runs, using software-only, or simple tools? [closed]

Are there any utilities or simple tools or techniques to easily test a CAT5 cable, to see how well was it installed? Cat5 is often installed by amateurs, but has been installed and "working" for ...
user135698's user avatar
16 votes
6 answers
31k views

Direct ethernet link between two servers

Say I had two servers which needed super-low latency (Database, file etc.). Would it be possible to directly connect the two servers with 10GbE, so each server had 1 (in the real world it would have 2)...
user avatar
15 votes
2 answers
86k views

eth0 NIC Link is Down repeating message in kernel log

I've noticed since few days ago that same repeating kind of messages occurs and I positively can say that nothing was intentionally changed (installed/uninstalled) in that period. here's sample of /...
Miloš Đakonović's user avatar
15 votes
1 answer
131k views

Where are ethernet errors logged?

Munin is showing me a graph like this: During that spike, I was unable to access my server through the eth0 port (I could access it through my IPMI port). I'm trying to figure out what happened, ...
Matt's user avatar
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14 votes
2 answers
3k views

Ethernet port plastic plug blocking port access on server

I have Dell Poweredge server with plastic blockers inside their ethernet ports. My problem is that I don't know how to take it out. You can see photo of the plug here: I tried to pull them out, or to ...
Marko's user avatar
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14 votes
1 answer
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What am I misunderstanding about calculating MTU?

Okay, I just finished resolving a jumbo frames issue between a few Xserves, a Netgear GSM7224, and a Drobo B800i. It turned out that the Xserves (Mac OS X 10.6.8 Server) and the Drobo B800i accept the ...
morgant's user avatar
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13 votes
4 answers
79k views

What happens when ARP Request comes from a different subnet?

What will happen when an ARP Request packet is sent from router1 to router2 in the following two cases? Will an ARP Reply be generated or the ARP Request packet be dropped? [router1]Intf1(20.0.0.1/...
gsinha's user avatar
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13 votes
5 answers
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A "splitter" is plugged into a patch panel... how does this still work?

I found a patch panel that splits off ethernet into multiple hosts as shown below. Can anyone explain how this might work?
makerofthings7's user avatar
13 votes
6 answers
21k views

Why does ethernet have a minimum frame size specified?

I think the minimum is 64 Bytes. Why is that minimum necessary?
user46749's user avatar
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12 votes
3 answers
47k views

What are the differences between channel bonding modes in Linux?

Under Linux you can combine multiple network interfaces into a "bonded" network interface to provide failover. But there are several modes, some of which do not require switch support. I'm not ...
hookenz's user avatar
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12 votes
3 answers
23k views

Which OSI-Layer does Ethernet belong to?

Just wondering.
user46749's user avatar
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12 votes
3 answers
15k views

What is, and what problems result in a 'carrier error'

I haven't found a really clear definition about what a 'carrier error' is within the context of the ifconfig output for a interface. I have been searching on Google and there really isn't a great ...
Zoredache's user avatar
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12 votes
1 answer
3k views

How does BER compare between different 10GbE physical media?

I am under the impression that you should use optical for 10GbE networking because of a BER (bit error rate) several orders of magnitude lower than copper. I'm pretty sure I previously found ...
MikeyB's user avatar
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11 votes
7 answers
3k views

Why straight-through ethernet cables?

I was explaining the difference between straight and crossover cables to my friend, and began wondering why on earth the early designers thought it was a good idea to use two different cable types? ...
zokier's user avatar
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11 votes
5 answers
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What is the disadvantage to cascading multiple switches?

I have to wire a new office for ethernet and the contractors want to charge atrocious amounts for each line running to each room in the office. It will be much cheaper for me to run a single line to ...
nganju's user avatar
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11 votes
6 answers
51k views

Does a single LACP channel over multiple switches increase redundancy?

I am curious for opinions, findings, or evidence that having multiple interfaces bonded using LACP to ports in multiple switches can increase redundancy. Previously bonded interfaces have always been ...
Sirch's user avatar
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11 votes
4 answers
8k views

How Many Ethernet Ports Can I Plug into a Single Computer

Given the fact there are Quad ports PCI/PCI express cards out there, I wonder how many of them (x4) can I plug into a single server? Perhaps the capacity bottleneck is the PCI bus, but I am not ...
Tzury Bar Yochay's user avatar
11 votes
2 answers
4k views

Why both T568A and T568B standards?

I've always wondered why there are the two different wiring standards for Cat5 UTP cabling. The only technical difference between the two is the green and orange pair are swapped, which accomplishes ...
user avatar
10 votes
3 answers
21k views

Which tool should I trust, mii-tool and ethtool don't show me the same values

Our hosting company provides all the switching infrastructure for our servers. The private network, connected to eth2 should be GbE. Recently I've been suspecting that something is misconfigured as ...
Dave Cheney's user avatar
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10 votes
7 answers
1k views

10 gigabit networks: Decieding between fiber and twisted pair

With a green field installation 10gbps network (no legacy equipment to support), why would one select one physical medium over the other? If left to impulse, I'd probably go with fiber optics over ...
Stu Thompson's user avatar
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10 votes
4 answers
8k views

Bridge to a used eth1 card without losing connection via that card

How can I set bridge between a real eth card on a remote machine without losing my ssh connection to that machine. I ssh into a machine via its eth1 iinterface. 172.168.40.2 255.255.255.0. I ...
Dave's user avatar
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10 votes
1 answer
3k views

Ethernet interface errors

My Ubuntu servers ethernet interface that connectes to ISP's multiplexer shows errors. Here is the snapshot: RX packets:204564288 errors:3193970 dropped:0 overruns:0 frame:3138402 ...
nixnotwin's user avatar
  • 1,553
9 votes
6 answers
2k views

What Ethernet twisted pair cable is best with a particular Ethernet switch

tl;dr I want the appropriate Ethernet cable for my Ethernet switch. How do I correlate Ethernet cabling speeds measured in hertz with an Ethernet switch "speed" measured in bits-per-second ...
JamesThomasMoon's user avatar
9 votes
2 answers
1k views

Why WakeOnLAN frames are sent to broadcast

I'm trying to figure out why wake on lan frames, in the simplest form, are designeted to the ethernet broadcast address (FF:FF:FF:FF:FF:FF) and not to the address of the machine to be powered on. ...
ApriOri's user avatar
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9 votes
3 answers
805 views

Cut network cables in Cisco 2960 PoE

We've a Cisco 2960e that services our VoIP phones and other things. I inherited it. The question I have is the cut cables: there are about four network connectors that are cut about 1cm away from the ...
Mei's user avatar
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9 votes
1 answer
376 views

LAN: What are the things to look at when arriving in a new place

My company is moving to a new place, and the only thing I know about the cabling is that there are cables and a patch panel. I have no idea about the cable speed, quality, and I'm a software guy with ...
CharlesB's user avatar
  • 585
9 votes
3 answers
23k views

How to test/debug bad network wiring?

I recently bought a place already wired with Cat 5E (8 ports, leading to a central closet). However attempting to get link, nothing works. On closer examination, it was obvious that the ends in the ...
Jack Lloyd's user avatar
9 votes
6 answers
25k views

Cheapest way to remotely control 120v outlet via ethernet? [closed]

What is the cheapest solution to be able to remotely power cycle a regular 120V outlet via ethernet? I've seen solutions like iBoot, but they're really expensive. Is there a cheap way to remotely ...
Robot's user avatar
  • 337
8 votes
6 answers
31k views

Is it possible to match an internal IP address to a switch port?

I'm trying to find a computer that has a certain IP address on our internal network. I have identified the computer name from DNS, but in this case it does not help me. Just wondering if I can ...
Brent 's user avatar
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8 votes
3 answers
64k views

Ethernet frames: what are the source and destination MAC addresses?

Imagine you have some network configured like this +-----+ +------+ +------+ +-----+ |comp.|---|bridge|---|router|---|comp.| | A |---| B |---| C |---| D | +-----+ +------+ ^ +------+...
F. P.'s user avatar
  • 199
8 votes
2 answers
3k views

How to fix the wiring mess that is my server room

So, I have inherited a mess of a server room. Rack 1: 5x 48 port patch panels; 140 in use | 3x 48 port switches Rack 2: 4x 48 port patch panels; 120 in use | 3x 48 port switches Rack 3: 3x 48 ...
Zero0ne's user avatar
  • 137
8 votes
1 answer
2k views

Can a limited broadcast packet contain destination MAC address other than FF:FF:FF:FF:FF:FF?

Can a limited broadcast contain destination MAC address other than FF:FF:FF:FF:FF:FF? If the answer is true, could you provide an example of a protocol that uses a packet like this. Ok, to elaborate ...
colemik's user avatar
  • 759
8 votes
0 answers
186 views

Shocking ESD damage, what more can be done? [closed]

Today, over $6,000 in networking equipment was destroyed. Remarkably, the main circuit SPD, power strips and APCs are fine. Ethernet seems to be the method of surge transmission, as everything ...
Justin's user avatar
  • 81
7 votes
11 answers
34k views

What is a good default IP address?

My company makes Ethernet devices. These devices have very little I/O. No keyboard, no video output. When the customer gets the device he must configure it for his network. I'd like to choose an ...
Robert Deml's user avatar
7 votes
8 answers
35k views

Effect of heat on CAT5 Ethernet Cabling

I am wondering what the heat rating is on standard CAT5 cabling or where I can check the heat rating of my cabling by using the information inscribed on the cabling? In my specific situation, I am ...
jmlane's user avatar
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