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178
votes
6answers
358k views

The slash after an IP Address - CIDR Notation

What does the "/16" means in here : "192.168.0.0/16", for example.
126
votes
8answers
64k views

Why do we need a 3-way handshake? Why not just 2-way?

The TCP 3-way handshake works like this: Client ------SYN-----> Server Client <---ACK/SYN---- Server Client ------ACK-----> Server Why not just this? Client ------SYN-----> Server ...
98
votes
6answers
618k views

How do you calculate the prefix, network, subnet, and host numbers?

Example: IP: 128.42.5.4 In binary: 10000000 00101010 00000101 00000100 Subnet: 255.255.248.0 How could you determine the prefix, network, subnet, and host numbers?
91
votes
12answers
28k views

IPv6 address space layout best practices

I'm comfortable with IPv4 address space allocations. By which I mean: Given services to plan for, or an organization to network, I have a good grasp of how to plan IP address space usage. (or at least,...
81
votes
20answers
69k views

When to choose fiber instead of twisted pair (copper)

These days both fibre and twisted pair are supporting 1000BASE-T and 10GBASE-T, but when do you choose one over the other? The obvious one is maximum cable length, but what are other factors which ...
80
votes
4answers
391k views

Why and how are Ethernet Vlans tagged?

I hear about VLAN tagging, but I don’t quite understand the concept. I know a trunk cannot accept untagged packets without configuring a native VLAN, and that access ports only accept untagged ...
74
votes
3answers
62k views

What's the difference between IKE and ISAKMP?

I've been building IPsec VPNs for years but to be honest I've never fully grasped the technical difference between IKE and ISAKMP. I often see the two terms used interchangeably (probably incorrectly)....
71
votes
5answers
30k views

Is “IPv10” a joke or a serious RFC draft?

Internet Protocol version 10 (IPv10) Specification The name is funny (IPv4 + IPv6 == IPv10), but the actual proposal looks strange (one more packet format to battle incompatibility between packet ...
60
votes
7answers
17k views

Why are IPv4 addresses running out?

I understand that we are running out (or ran out already?) of IPv4 addresses, but I don't really understand why that is. Right now, every home has its own IPv4 address (dynamically assigned, but still,...
57
votes
9answers
20k views

Is 30 Mbit/s fibre faster than 30 Mbit/s copper? [duplicate]

A network engineer professional is overseeing our town's installation of fibre and was explaining how much faster it was. I pointed out that it wasn't a silver bullet as you had to pay the provider ...
55
votes
14answers
190k views

Multi-Mode and Single-Mode Fiber Differences

When cabling a network using fibre, what is the difference between single-mode and multi-mode fibre? When should I be using one or the other? Are there compatibility and/or speed concerns with either?
54
votes
10answers
95k views

Why only port 80 for web services?

Why isn't it sensible to dedicate more than one TCP/IP port to http? Although admittedly naive, isn't it somehow intuitive to think that server performance could somehow be increased?
50
votes
4answers
269k views

PING port number?

Yesterday interviewer ask me what is port number for ping and which protocol ping use TCP/UDP. After interview I search on internet and found different different results someone says ICMP uses Port 7,...
48
votes
24answers
91k views

How can I see which switchports are not in use?

On a large Cisco Catalyst switch stack, almost all switchports are patched. I need to identify the ports, which are not in use, for connecting further devices. Following switchports, cabling, patch ...
47
votes
13answers
120k views

Does having a longer Ethernet cable slow your connection?

As the title suggests, Does a longer Ethernet cable slow your connection down?
46
votes
8answers
4k views

Network Neutrality, how does it actually work, technically speaking?

In this NPR article there is mentioned 'network neutrality' with very little details on what it actually is or how it actually works. I tried researching it on my own but I get a lot of non-technical ...
44
votes
5answers
8k views

Why do we need IPv6?

This will be a kind of newbie question but I am not quite sure why we really need IPv6. AFAIK, the story is as follows: In the olden days, when computers were not plentiful, 32 bit IP addresses were ...
43
votes
15answers
78k views

How can I diagnose a bridging (ethernet) loop?

Given that spanning tree has failed (or you don't have any spanning tree) and get an ethernet loop, what's the best way to diagnose where the problem is? Which switch?, which cable? and so on.
43
votes
5answers
13k views

Is Ethernet port blinking really useful?

This question may not look as an important one and in fact it is just out of curiosity. But to the point: Is Ethernet port blinking really useful? Ethernet ports usually have two light indicators ...
43
votes
4answers
44k views

Why is fe80::/10 reserved for link local addresses when fe80::/64 is actually used?

I understand that IPv6 reserves the fe80::/10 prefix for link local addresses, and that hosts select an address with a /64 prefix. Why is the reserved space so much larger than what is actually used ...
42
votes
2answers
243k views

Why do I see a RST, ACK packet instead of a RST packet?

Looking in Wireshark, I often see TCP Streams end with a RST, ACK packet instead of a RST packet. Anyone know why this is? An example of what I see: SYN SYN, ACK ...data... RST, ACK Wireshark is ...
41
votes
1answer
4k views

Why is such a large IPv4 range assigned to localhost?

Why is so large a range of IPv4 addresses assigned to localhost, (127.0.0.0 through 127.255.255.255)?
40
votes
4answers
35k views

Bits per second vs. Packets per second

Lately I have been checking datasheets of several switch models from diferent vendors. For a given switch, vendors publish a couple of figures that I think are a measure of the capacity/performance of ...
39
votes
4answers
21k views

How to check for a valid public IP address?

Consider a list of IP addresses as provided by the X-Forwarded-For HTTP header: 10.0.0.142 , 192.168.0.10 , 212.43.234.12 , 54.23.66.43 I would like to know which is the first publicly-accessible ...
39
votes
11answers
35k views

How do you prevent rogue wireless access points on a network?

Depending on what type of traffic is going over the network, it's often not feasible that an employee brings a wireless router and sets it up into your network. This is because often, they are not or ...
38
votes
5answers
34k views

Why was the MTU size for ethernet frames calculated as 1500 bytes?

Is there any specific calculation that that was done to arrive at this number, and what were the factors that were taken into consideration for that calculation.
37
votes
5answers
7k views

Why does ipv6 specify 128 bit address, when there are only 48 bits in MAC addresses?

So, I'm reading for a networking exam, and I'm just wondering if I have missed something basic. Is there a need for more ip addresses than MAC addresses, and how would a node with one network adapter ...
37
votes
5answers
37k views

What is the meaning / origin of the terms north-south and east-west traffic?

In context of networking I keep running into statements such as "east-west traffic is larger than north-south traffic". I have tried a bit of googling but have not been able to locate an authoritative ...
37
votes
8answers
27k views

How would a router with /32 WAN subnet mask communicate with the rest of the Internet?

I'm studying networking and one of the most basic things I've been told countless times is that computers can't communicate outside of their own subnet. On the other hand, by looking at some ADSL ...
36
votes
3answers
108k views

How does gratuitous ARP work?

Does gratuitous ARP work like a normal ARP request? Why is gratuitous ARP used for HSRP?
35
votes
13answers
51k views

Difference between Subnet and VLAN?

You often hear the terms Subnet and VLAN used interchangeably. With the ubiquitous nature of IP these days, when are the two not considered roughly the same from a high-level, understanding that ...
35
votes
3answers
49k views

Does a traceroute/tracert show every hop, or does it skip/hide some details of the path?

I am currently in college for a bachelor's degree in Network Engineering, and one of my Professors explained in class that a traceroute that shows, for example, 15 hops is actually abstracting the ...
35
votes
3answers
2k views

Why do network taps have four ports?

I've been looking at a hardware network tap like this one to replace a pseudo-permanent SPAN that's been running on a Catalyst switch. All the taps I find have four interfaces: A, B, and two output ...
34
votes
5answers
8k views

Why are Ethernet Standards written in the form of 10/100/1000? Why not just 1000?

I always have seen Ethernet Standards written in the form "low value/middle value/max value", and always wondered why. Won't a router just reproduce a lower speeds anything below its maximum if it's ...
34
votes
3answers
39k views

Why 192.168.*.* for local addresses? [closed]

The standard I've seen so far is to use 192.168.*.* IP addresses for devices on the local network. Why this combination? If it were me, I would have chosen something simpler, like 1.0.*.*. What's the ...
33
votes
3answers
7k views

Why are IPv4 addresses 32-bit?

Many moons ago, when I was just a wee bairn commencing my career, I had a job interview for a low-level developer role. Having at that time just learned how CIDR was implemented, I was keen to show ...
33
votes
3answers
96k views

What is the difference between MSS and MTU?

I'm confused as to the difference between maximum segment size and a maximum transmission unit. Can someone please explain in relation to layers 2 and 3? If I had a packet of 800 bytes in the ...
33
votes
1answer
66k views

Using SNMP to retrieve the ARP and mac-address tables from a switch

I would like to get ARP tables from a switch to a syslog-ng server that has been set up on Ubuntu Server 12.04 LTS. I have read about SNMP and I know the server will act as a manager and the switch as ...
33
votes
2answers
79k views

What is the actual size of an Ethernet MTU

I think I might be getting confused with terminology surrounding MTU. This definition from Wendell Odom's CCNA book on MTU: The IEEE 802.3 specification limits the data portion of the 802.3 frame ...
33
votes
5answers
3k views

Better Enterprise Multihoming

I would like to get some opinions regarding ways in which I can improve a BGP dual-provider, dual-router design. Each provider supplies a /24 public subnet. I will refer to the routers, circuits, ...
32
votes
8answers
7k views

How can I stop an intruder plugging into an Ethernet wall socket getting access to the network?

Is MAC address filtering the most suitable option to prevent someone connecting their own device to the network by plugging into Ethernet wall sockets? What if they unplug a device and clone its MAC?
29
votes
5answers
5k views

Why do we still use Ethernet?

There's no doubt the vast majority of Ethernet frames are transporting IP packets. I know there are various other protocols that can be transported over Ethernet as well, but those can be transported ...
29
votes
8answers
33k views

What are the downsides of OpenVPN?

I have been seeing so many people always wrestling with IPSec, and many other secure VPN technologies. I, for one, have always simply used OpenVPN, with beautiful and simple and versatile results. I'...
28
votes
8answers
9k views

What does the 1000M on a network switch mean?

I thought the 1000M ment that it was capable of Gigabit speed but it isn't, I noticed then that it stated 10/100Mbps. What does this 1000M mean?
28
votes
5answers
24k views

Are CoS and QoS ever interchangeable?

The terms CoS & QoS are often used interchangably, particularly in the SP space when providing "Class of Service profiles" etc as part of Product sets. I'm looking to re-affirm my understanding ...
28
votes
2answers
48k views

What is the meaning of the CRC counter on a cisco device?

If the CRC counter of an interface is high, normally it's a bad sign, but why? If the count is high, what does this mean technically? What can cause this counter to go up? On which layer in the OSI ...
27
votes
5answers
12k views

OSI Model and Networking Protocols Relationship

When most networking students first learn about the OSI model, they spend a great deal of time trying to figure out which layer of the model a particular protocol fits into. We get a lot of questions ...
26
votes
3answers
5k views

What is “oversubscription” in Networking?

I have a question about oversubscription in networking. I read a lot of documentation but I still don't understand what it means. I read the following on the Cisco website, oversubscription of the ...
26
votes
7answers
7k views

Does it make sense to deploy OSPF across Metro Ethernet?

When I need to connect a number of branch sites to one another for a customer, I typically recommend an MPLS VPN through a trusted carrier. The CE at each site speaks BGP with its upstream PE and each ...
25
votes
7answers
6k views

Why isn't UDP with reliability (implemented at Application layer) a substitute of TCP?

TCP provides reliability at transport layer while UDP does not. So, UDP is fast. But, a protocol at application layer can implement reliable mechanism while using UDP. In this sense, why isn't UDP ...

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