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82 votes
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Why does DHCP use UDP and not TCP?

DHCP cannot use TCP as the transport protocol because TCP requires both end-points to have unique IP addresses. At the time a host is required to use DHCP, it does not have an IP address it can source ...
Mario Jost's user avatar
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47 votes

Why can I traceroute to this IP address, but not ping?

On a similar question here Luke Savage explained it perfectly: Traceroute is not a protocol itself, it is an application and the protocols used depends on the implementation your are using. Primarily ...
naïveRSA's user avatar
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30 votes
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What is the use of NTP server when devices have accurate time?

Internal clocks are only so accurate. With an uptime of months or sometimes years, your nodes can drift considerably from correct time. Also, some nodes lack a battery-powered RTC, so they default to ...
Zac67's user avatar
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28 votes
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Why is Packet Size Limited?

Why we don't just send one single packet? why we need to split content into multiple pockets (ignoring size limit). That would just lead back to circuit-switched networks like the original PSTN (...
Ron Maupin's user avatar
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26 votes

Why does DHCP use UDP and not TCP?

Since the source has no IP address (0.0.0.0) and the destination is everyone (255.255.255.255), it's hard to see how you would identify a particular session. But even if you could, what would be the ...
Ron Trunk's user avatar
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26 votes

What is the use of NTP server when devices have accurate time?

Your question is based on a number of inaccurate assumptions. First off, not all systems actually have an internal clock. The Raspberry Pi is a trivial example of such a system, but there are many ...
Austin Hemmelgarn's user avatar
22 votes
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What is the least possible separation of two NICs with the same MAC address?

Suppose you have two NICs with the same MAC address, but not necessarily the same IP address. You can't have that within the same link-layer segment. Identical MAC addresses will disable reliable ...
Zac67's user avatar
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22 votes
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Why in TCP the first data packet is sent with "sequence number = initial sequence number + 1" instead of "sequence number = initial sequence number"?

This "bump the sequence# forward by 1 for SYN and FIN flags" is sometimes referred to as the "Phantom Byte" -- since no actual byte of data is sent, but the presence of the SYN or ...
Eddie's user avatar
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21 votes

Why does DHCP use UDP and not TCP?

There are multiple reasons why TCP wouldn't work for DHCP(v4.) First of all, TCP is connection-oriented. A TCP connection is defined between two particular hosts. However, when a DHCP client first ...
reirab's user avatar
  • 469
21 votes

Is `0.0.0.0/1` a valid IP address? If so, what does it indicate?

Zac is muddying the waters by excluding 0.0.0.0/8 and 127.0.0.0/8. While addresses in those two prefixes should never appear on the wire, 0.0.0.0/1 does cover them. The slash notation works the same ...
Ricky's user avatar
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19 votes

Why can I traceroute to this IP address, but not ping?

To add to @naïveRSA's answer, if there's filtering/firewalling in the path one could also have the situation where an ICMP "echo reply" (ping) packet is blocked, but an ICMP "time exceeded" (tracert) ...
Arjan's user avatar
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18 votes
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what is difference between MSS and Window Size?

The Maximum Segment Size is the largest TCP segment that can be transported in a single IP packet. It is derived from the Maximum Transfer Unit (MTU) minus IP header overhead minus TCP header overhead....
Zac67's user avatar
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16 votes
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Is `0.0.0.0/1` a valid IP address? If so, what does it indicate?

0.0.0.0 is no valid IP address, regardless of prefix length. 0.0.0.0/0 as prefix matches any address, so it's used for the default route. Also, 0.0.0.0 is the unspecified address used in many APIs, ...
Zac67's user avatar
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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
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13 votes

Do network adapters read incoming bits in a single stream?

Let's ignore the Gigabit part for now, and focus on your "2 devices are sending at the same time" part for a bit. On shared media, this can actually happen and be a problem. Most wireless ...
jcaron's user avatar
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13 votes
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Why do IPv6 unique local addresses have to have a /48 prefix?

The requirement exists to prevent collisions. This is a bit more important than most people recognize. Even if you have systems which currently don't communicate with other systems over the internet ...
kasperd's user avatar
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13 votes

What is the use of NTP server when devices have accurate time?

A slightly different perspective than the other answers, in the context of information security. We do not really care about how time is precise. What we want to make sure of is that time is the same ...
WoJ's user avatar
  • 243
12 votes

Do network adapters read incoming bits in a single stream?

This particular case is a complex one. Regarding 1000baseT. First: when we say in general that two devices are transmitting at the same time, they are not normally actually sending bits at the same ...
jonathanjo's user avatar
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12 votes
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Encapsulation: can a higher protocol encapsulate a lower protocol?

Yes, encapsulation hide the details of what is encapsulated and doesn't really care about the payload nature. VxLAN is a sensible example of this, with layer2 (VLAN) being encapsulated in layer 4 (UDP)...
JFL's user avatar
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12 votes
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Does an unmanaged switch deal with IP addresses at all

An unmanaged switch doesn't use/care for/understand IP addresses at all. A managed L2 switch uses IP addresses for management only. Some L2 switches also support limited L3/IP functionality like ACLs. ...
Zac67's user avatar
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12 votes

Why is Packet Size Limited?

In general there are several reasons to limit packet size. A larger packet has a longer transmission duration, which means it ties up the line for longer, increasing jitter for other (potentially ...
Peter Green's user avatar
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11 votes
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is there a difference between Latency and speed ?

You can have high speed, but if you have to go through a long distance then you have also high latency. Let's use Los Angeles and London to understand this: The distance from Los Angeles to London ...
jcbermu's user avatar
  • 4,497
11 votes

Why do IPv6 unique local addresses have to have a /48 prefix?

When companies merge or set up an extranet to communicate, it has proven difficult with IPv4 Private addressing because the companies often use the same or overlapping address space, and that requires ...
Ron Maupin's user avatar
  • 99.9k
11 votes

APIPA and LAN Broadcast Domain

Welcome to Network Engineering! If I understand your question, it's "can my network devices communicate on a single LAN using APIPA addresses? The answer is YES. APIPA addresses are in the 169....
Ron Trunk's user avatar
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11 votes

Can anyone explain to me the difference between the PSH and URG flags in TCP segment?

Jeremy Stretch has a good article on this. This is where the PSH flag comes in. The socket that TCP makes available at the session level can be written to by the application with the option of "...
Ron Trunk's user avatar
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10 votes
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Why can't we create a hierarchy of hubs?

To add to Ron's point - a hub (...or a number of hubs) basically models the behavior of the original Ethernet, which is to say basically a big piece of coaxial cable. If one station transmits, all ...
rnxrx's user avatar
  • 6,124
10 votes

session layer in OSI model

The OSI model is just a model, and OSes do not implement the OSI model. Modern OSes are much closer to the IP Services model, but it, too, is just a model, and things in the real world do not always ...
Ron Maupin's user avatar
  • 99.9k
10 votes

session layer in OSI model

The session layer sits on top of the transport layer. In the case of a connection-oriented services, the transport service is responsible for providing reliable flow-controlled end-to-end data ...
Bruno Rijsman's user avatar
9 votes

is there a difference between Latency and speed ?

Latency is really just distance: the time it takes for the beginning of a bit to leave here and arrive there. (As perfectly explained in the other answer) this is entirely unrelated to speed: it ...
jonathanjo's user avatar
  • 16.3k
9 votes

Can anyone explain to me the difference between the PSH and URG flags in TCP segment?

Suppose the receiving buffer has already some data to be processed by the application. A segment with the PSH flag set to 1 is sent now. The sending buffer will not wait to be filled, instead, it will ...
avistein's user avatar

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