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61 votes
Accepted

Is Ethernet port blinking really useful?

Yes, blinkenlights are your friends! The lights can be very helpful when diagnosing problems - especially when dealing with non-managed switches or remote diagnosis with unexperienced users. Rule of ...
Zac67's user avatar
  • 85.8k
43 votes

Does having a longer Ethernet cable slow your connection?

No, it will not slow down a connection, but you need to be aware of the maximum length of a copper connection which is 100 meters. This needs to include the length of your patching cable from the host ...
SleepyMan's user avatar
  • 2,016
35 votes

Does having a longer Ethernet cable slow your connection?

For all practical purposes, there will be no effect on the speed of your connection. There will be a very insignificant amount of delay due to long cables. This won't affect the maximum speed of ...
Cort Ammon's user avatar
35 votes
Accepted

Why is the CAM table in a switch called CAM table and not MAC table even though it holds MAC addresses?

CAM (Content Addressable Memory) is memory that can be addressed by content, rather than a numeric memory address. You can look up the interface by presenting the memory with the MAC address. This is ...
Ron Maupin's user avatar
  • 100k
33 votes

What is "industrial ethernet"?

Like many IT terms, it's primarily a marketing term. It usually refers to Ethernet switches that are designed for harsh environments (i.e. industrial). The switches may have higher temperature ...
Ron Trunk's user avatar
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32 votes
Accepted

Why would a switch be connected to a router using three ethernet cables?

The switch is probably a managed switch on which multiple VLANs are configured. The three cables between the router and the switch are used to provide inter-VLAN routing. Another possibility is ...
dr_'s user avatar
  • 1,299
32 votes
Accepted

Why does Ethernet use EtherType field to determine what type of packet is in a frame instead of just looking at the packet header?

The receiver has to look at the Ethernet frame to decide its contents, which might be DECnet, Appletalk or many other things -- Internet Protocol is only one of many protocols running on top of ...
jonathanjo's user avatar
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31 votes
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Why does an Ethernet cable have four pairs?

10BASE-T saw first light as StarLAN that made use of the already existing twisted-pair category 3 telephone cabling (instead of the dedicated coax that 10BASE5/2 required). That multi-purpose cabling ...
Zac67's user avatar
  • 85.8k
29 votes
Accepted

How can a TCP window size be allowed to be larger than the maximum size of an ethernet packet?

The TCP window size is generally independent of the maximum segment size which depends on the maximum transfer unit which in turn depends on the maximum frame size. Let's start low. The maximum frame ...
Zac67's user avatar
  • 85.8k
29 votes

Is Ethernet port blinking really useful?

My very first step in troubleshooting network issues is this: "Are there winky-blinky lights?" It's the network port equivalent of asking someone if their computer and/or monitor is actually ...
Dustin Kreidler's user avatar
27 votes
Accepted

What's the point of auto-sensing 10/100 Mbit/s Ethernet?

Some devices could only run at 10 megabit/s, so the device at the other end would autosense the speed to match. If a device that has a maximum speed of 10 Mbit/s is connected to a 10 Mbit/s /...
Ted Quanstrom's user avatar
23 votes
Accepted

Why can a packet sniffer in an Ethernet LAN obtain all packets sent over the LAN?

In classic (obsolete) Ethernet, a shared wire or repeater hubs were used. So, each node physically receives every frame sent within the broadcast domain (also the collision domain in this case). ...
Zac67's user avatar
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22 votes
Accepted

Minimum ethernet frame is 64 bytes, Why the payload must be padded to at least 46 bytes

The entire frame has to be at least 64 bytes. This is not just the payload, this includes the headers and the frame check sequence. The FCS takes up 4 bytes at the end. An Ethernet header consists of ...
alex.forencich's user avatar
20 votes
Accepted

Is the MTU of Ethernet still 1500 Bytes?

Standard maximum payload size for Ethernet is still 1500 bytes. While the maximum frame size has grown slightly, from 1518 to 1522 byte with 802.1Q and further with 802.1AD, the payload, "MAC client ...
Zac67's user avatar
  • 85.8k
20 votes

What is "industrial ethernet"?

Industrial Ethernet is the use of Ethernet in an industrial environment with protocols that provide determinism and real-time control. Protocols for Industrial Ethernet include EtherCAT, ...
infra's user avatar
  • 3,138
19 votes

How does gratuitous ARP work?

A Gratuitous ARP is an ARP Response that was not prompted by an ARP Request. The Gratuitous ARP is sent as a broadcast frame, as a way for a node to announce or update its IP to MAC mapping to the ...
Eddie's user avatar
  • 15.1k
19 votes

Does having a longer Ethernet cable slow your connection?

Sort of, to a very tiny extent. The longer your cable, the higher latency you experience - gamers call this "ping" time. However, the effect is about one nanosecond per foot of cable, which is ...
pjc50's user avatar
  • 321
18 votes
Accepted

Why do I need a crossover cable to connect devices of the same type?

When you need crossover cables is often explained, but why is seldom explained. It has to do with the copper (often referred to as Ethernet) wire itself. In copper wiring, there are four pairs of two ...
Eddie's user avatar
  • 15.1k
18 votes

Why does PPP need an underlying protocol?

Why does PPP need a wrapping protocol? PPP is not a layer-1 protocol, so it needs a layer-1 protocol to carry it. Protocols like ethernet are both layer-1 and layer-2 protocols, so PPP can use ...
Ron Maupin's user avatar
  • 100k
17 votes
Accepted

What do the positive and negative (+/-) transmit and receive pins mean on Ethernet cables?

Twisted pair uses differential signaling - in a pair, one wire is always the negative/complimentary signal of the other. In the simplest example, Transmit+ > Transmit- (higher voltage level) means 1 ...
Zac67's user avatar
  • 85.8k
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
  • 2,269
16 votes
Accepted

How exactly does an Ethernet collision happen in the cable, since nodes use different circuits for Tx and Rx?

To understand this you need to understand the historical context. Originally Ethernet used a shared coaxial cable. Only one device could successfully transmit on this at a time. If two devices ...
Peter Green's user avatar
  • 13.4k
15 votes

Why is the CAM table in a switch called CAM table and not MAC table even though it holds MAC addresses?

CAM - Content Addressable Memory, referring to the memory used for the MAC address table. It works kind of reverse from RAM, you address it by giving it content and it returns you the address where ...
manish ma's user avatar
  • 1,644
15 votes

How exactly does an Ethernet collision happen in the cable, since nodes use different circuits for Tx and Rx?

A hub is really just a powered cable that repeats every signal it receives on one interface to all the other interfaces. If two devices transmit at the same time to the receive of the hub interfaces, ...
Ron Maupin's user avatar
  • 100k
14 votes

What's the point of auto-sensing 10/100 Mbit/s Ethernet?

Remember 10Mbps came first, then 100Mbps, then 1000Mbps. The advantage of supporting multiple speeds and automatically switching between them is you can upgrade your network gradually without having ...
Peter Green's user avatar
  • 13.4k
14 votes
Accepted

The maximum length of CAT6 for intranet network?

Per the ANSI/TIA/EIA 568, Commercial Building Telecommunication Standard, UTP cabling is limited to 100 meters. That length assumes up to 90 meters of solid-core (better performance, but fragile) ...
Ron Maupin's user avatar
  • 100k
14 votes

What does the naming convention for Ethernet standards mean: 1000BASE-T, BASE-TX, BASE-SX, etc.? What is the meaning of the components of the name?

BASE indicates baseband signaling - there is no modulated carrier, the frequency starts near zero and extends to a certain cut-off frequency. BROAD indicates broadband modulation - there is a wide ...
Zac67's user avatar
  • 85.8k
14 votes

Do network adapters read incoming bits in a single stream?

That depends. While many Ethernet PHYs transmit data in a purely serial fashion (e.g. 100BASE-TX, 1000BASE-SX, 10GBASE-SR), some split the data stream into multiple lanes that are transmitted in ...
Zac67's user avatar
  • 85.8k
14 votes

Why maximum length of IP, TCP, UDP packet is not suit?

IP protocol build on Ethernet or something, Why an IP packet can be 65535 bytes when Ethernet can only send 1500 bytes? Ethernet is one of several physical layers which can be used to to transport IP ...
Steffen Ullrich's user avatar
14 votes

Why can I send 127.0.0.1 to 127.0.0.0 on my network?

we changed our FW so that the fixed IP addresses are 127.0.0.0 and 127.0.0.1. Generally, that's a really bad idea. It is in violation of RFC 1122: (g) { 127, <any> } ...
Zac67's user avatar
  • 85.8k

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