77 votes

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 ...
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  • 1,635
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 ...
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  • 740
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 (...
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  • 93.5k
25 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 ...
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  • 62.4k
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 ...
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  • 70.5k
20 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 ...
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  • 459
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) ...
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  • 331
17 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....
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  • 70.5k
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 ...
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  • 70.5k
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 ...
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  • 653
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 ...
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  • 764
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 ...
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  • 15.9k
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)...
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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. ...
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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 ...
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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 ...
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  • 4,417
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 ...
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  • 93.5k
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....
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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 ...
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9 votes
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What precisely is a link?

There is no single precise definition for a "link". A link can be a physical layer connection, two ports connected by a cable. A link can also be understood as general connectivity by data link ...
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9 votes
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What's the DF flag mean in IP packet header?

The IPv4 DF flag means that an intermediate host (router) cannot fragment the packet if necessary, and it would then need to drop the packet and can send an ICMP message stating that. RFC 791, ...
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  • 93.5k
9 votes

Encapsulation: can a higher protocol encapsulate a lower protocol?

Consider a package delivery service, like UPS or DHL. They don't care what's inside the box - they just make sure it gets to its destination. Similarly, the protocol doesn't care what the payload ...
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  • 62.4k
9 votes

Why can't we create a hierarchy of hubs?

Are uplink ports simplex ? Hub interfaces are simply hub interfaces, there really are no uplink interfaces. Why is it not possible for the hubs to read from the uplink port and broadcast the frame ...
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9 votes
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Different Behaviour of a Router

You don't have to add anything in the 1st case because it's automatic. As there's only one router, it obviously knows everything directly connected to it. When that becomes more than one router, some ...
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  • 29.6k
9 votes

Different Behaviour of a Router

Why in the first case, There was no need of adding Routing table Because it's not really true that there was no need of adding a routing table. When you configured the IP addresses and subnets of the ...
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  • 191
8 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 ...
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  • 15.9k
8 votes
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Packet Collison Avoidance

A wireless network is only a single, shared medium with a limited total bandwidth. The more clients compete for bandwidth the smaller each slice gets. Additionally, the simple presence of clients ...
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  • 70.5k
8 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 "...
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8 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 ...
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