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

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70
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17answers
28k 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 ...
45
votes
2answers
29k views

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 ...
34
votes
6answers
16k views

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?
19
votes
3answers
761 views

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 ...
18
votes
7answers
92k views

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?
17
votes
2answers
7k views

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 ...
14
votes
5answers
6k views

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 ...
13
votes
2answers
351 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 ...
13
votes
1answer
2k views

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 ...
12
votes
1answer
703 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 ...
11
votes
6answers
1k 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? ...
11
votes
5answers
2k views

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?
10
votes
7answers
919 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 ...
10
votes
2answers
2k 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 ...
9
votes
6answers
3k 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 ...
9
votes
3answers
404 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 ...
9
votes
2answers
477 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. ...
9
votes
1answer
291 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 ...
9
votes
4answers
1k 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 ...
9
votes
3answers
2k views

Why does ethernet have a minimum frame size specified?

I think the minimum is 64 Bytes. Why is that minimum necessary?
9
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4answers
5k 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 ...
9
votes
1answer
2k 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 ...
8
votes
3answers
5k 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 ...
8
votes
2answers
2k 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 ...
7
votes
7answers
15k 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 ...
7
votes
5answers
7k views

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 ...
7
votes
3answers
2k 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 ...
7
votes
3answers
1k 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 ...
7
votes
1answer
829 views

What is theoretical maximum of TCP traffic over GigaEthernet?

What is theoretical maximum of TCP traffic over GigaEthernet ?
7
votes
1answer
690 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 ...
7
votes
1answer
3k views

How do I poll the ARP table with an SNMP OID?

When you need to poll the ARP table of a device, conventional wisdom is to use ipNetToMediaPhysAddress or atPhysAddress; however, both of these OIDs are deprecated according to Cisco's documentation. ...
6
votes
2answers
960 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 ...
6
votes
1answer
460 views

FCoE, on any Ethernet switch?

I understand the concept of FCoE. I have looked at the Wikipedia page and looking at the layer 2 frame diagram, it looks like FCoE really should "just work" on any Ethernet switch, but is this really ...
6
votes
4answers
253 views

What's better for performance: rack-to-endpoint or rack-to-switch-to-endpoint

When putting Ethernet cables for an office there are two ways to put the cables: From the rack patch panel a cable to each end-point (workstation). From the rack patch panel a cable to each ...
6
votes
4answers
15k views

How can I reset the statistics reported by ifconfig?

I'm trying to trace a possible link problem on one of our servers' ethernet interfaces. ifconfig reports a large number of error packets. I've asked the hosting company to make some changes to the ...
6
votes
5answers
4k views

Cheapest way to remotely control 120v outlet via ethernet?

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 ...
6
votes
1answer
247 views

Is it possible to detect rx+ rx- tx+ tx- wires with multimeter?

Is it possible to detect rx+ rx- tx+ tx- wires of an ethernet cable having only a part of cable with multimeter or somehow else? I mean the situation when you have just one side of the ethernet ...
6
votes
1answer
295 views

Why do metal fasteners (e.g., staples) around ethernet cables impair network performance? [closed]

At one point I was living in a 6 bedroom home that had cat 6 ethernet wired to each bedroom. The internet connection to the bedrooms furthest from the cable modem had terrible performance, though I ...
6
votes
6answers
3k views

Expected throughput in Gigabit network

I've been doing some tests with iperf to measure the network throughput between 2 servers (connected back-to-back thru a crossover cable) and I only see 180 to 200 Mbps. I tried different cables and ...
6
votes
5answers
753 views

Keystone vs. Feed-Thru vs. Punchdown Patch Panels

We have a new office that needs some ethernet wiring done to a cabinet in a closet (no more than 48 cables). There seems to be so many options now. I'd traditionally terminate everything at a ...
5
votes
9answers
6k views

Why does 1GBit card have output limited to 80 MiB?

I'm trying to utilize maximal bandwidth provided by my 1GiB network card, but it's always limited to 80MiB (real megabytes). What can be the reason? Card description (lshw output): description: ...
5
votes
2answers
362 views

Why ethernet cables must be ended with specific arrangement

I just accepted that ethernet cables CAT 5 and more must be ended with specific arrangement. I learned when I ending my cables to take attention that either end must be in same arrangement(568A or ...
5
votes
3answers
1k views

Multiplexed 1 Gbps Ethernet?

Is it possible to multiplex two (or more) 1Gbps Ethernet into a single logical connection? Is it common place? Advisable? Stupid? Other considerations? I ask because my hosting partner's network ...
5
votes
3answers
336 views

Gigabit uplink versus regular switch port

I have a 10/100/1000 24-port + 2 mini-GBIC ports switch which I have to link to other switches. Unfortunately, on the other switch all the mini-GBIC slots are taken up, so all I can do is link them ...
5
votes
1answer
713 views

What does a magic packet look like?

What does a magic packet contain and what options does the protocol provide?
5
votes
1answer
1k views

Client not getting address from some DHCP server

I'm working on some embedded devices which should be connected to a large network (hundreds of devices) via ethernet and that should be configured using the DHCP protocol. What I'm experiencing is ...
5
votes
1answer
195 views

Is it possible to receive dhcpdiscover from ff:ff:ff… mac address?

Today I saw this record in my log files: dhcpd: DHCPDISCOVER from ff:ff:ff:ff:ff:ff via vlan400: network 40.60.100.10/24: no free leases ...
5
votes
4answers
302 views

Are there any statistics on the number of networks using Ethernet Jumbo frames?

I am often told by Sales Engineers and Product managers that a Layer 2 Ethernet service is not fit for purpose if the maximum supported frame size is in the 1518-1522 range (enough to support standard ...
4
votes
10answers
606 views

Windows packet sniffer

What packet sniffer would you recommend using, especially in a 50+ workstation environment? I am a fan of Wireshark but it's got quite a bit of security history. Is there something better?
4
votes
11answers
7k 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 ...