Questions tagged [ip]

For questions about Internet Protocol (IP), an OSI layer-3 network protocol. There are two current IP versions in use: IPv4 and IPv6.

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129 votes
6 answers
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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?
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72 votes
5 answers
32k 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 ...
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70 votes
5 answers
20k 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,...
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46 votes
5 answers
9k 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 ...
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40 votes
4 answers
24k 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 ...
38 votes
2 answers
158k views

TCPDump - Filter by MAC Address

I would like to display all traffic for or from a specific MAC address. For that I tried sudo tcpdump host aa:bb:cc:11:22:33 It does not work and returns me an error tcpdump: pktap_filter_packet: ...
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36 votes
3 answers
16k views

Why does the IPv6 header not include a checksum?

It is stated in Wikipedia that an IPv6 header does not include a checksum. What are the reasons that were behind this decision?
29 votes
2 answers
17k views

Is multicast on the public internet possible? And if yes: How?

Is it possible to use multicast on the public internet? If yes: How? Are special IP addresses required and where do you get them from?
26 votes
3 answers
6k views

Will IPv6 make private IP addresses become obsolete?

Will it still be useful to make use of private IP addresses with IPv6?
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24 votes
7 answers
9k views

Does CIDR really "do away" with IP address classes?

I am still struggling to understand to what extent CIDR really renders IP address classes obsolete. Here's what I understood so far: It's ridiculously inefficient (and impossible, too) to assign ...
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22 votes
6 answers
6k views

What stops someone from configuring their network with IP addresses they do not own?

Here's the scenario. I was picturing a university that bought a range of IP addresses. I think their network would still be connected to an ISP (right?), but they'd have freedom to configure stuff the ...
22 votes
1 answer
13k views

What is the practical use of a /31 CIDR or 255.255.255.254?

With a /31 subnet, we are left with zero usable hosts. The /31 subnet only has two hosts - one for network and another for broadcast. I am wondering why would someone ever use this subnet.
20 votes
2 answers
6k views

What is the difference between 0.0.0.0 and a loopback IP address?

From the book Computer Network: The IP address 0.0.0.0, the lowest address, is used by hosts when they are being booted. It means ‘‘this network’’ or ‘‘this host.’’ ... All addresses of the ...
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20 votes
3 answers
34k views

What is Link-local addressing?

I noticed that IPv4 reserved 169.254.0.0/16 and IPv6 reserved fe80::/10 for link-local addressing. While I saw IPv6 uses both fe80::/10 address and my private IPv6 address fd00::12 to communicate with ...
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19 votes
9 answers
12k views

Why are IP addresses given to each interface and not device? What would the implications of that be?

Why do we need to give IP addresses to each interface? Wouldn't giving one to each device be enough?
19 votes
5 answers
4k views

How is 8 bits sufficient for the TTL in an IP header?

The TTL (Time to Live) is an 8-bit field in the IPv4 header. It can take any value from 0 to 255. If this means that the packet can take a maximum of 255 hops (routers) on its way to its destination, ...
19 votes
3 answers
9k views

What do TCP/UDP add to "raw ip"?

I know that both TCP and UDP are built on top of IP, and I know the differences between TCP & UDP, but I'm confused about what exactly "raw ip" is. Would it be fair to say that TCP & UDP both ...
18 votes
6 answers
33k views

IPv4 Segment 100.64.0.0/10

I recently got to know that IP segment 100.64.0.0/10 has been reserved by IANA for 'Shared Address Space'. My question is: Can I use the IP Segment 100.64.0.0/10 as a private range in my network (...
17 votes
3 answers
3k views

Is it possible to specify a route for a packet to take?

So, AFAIK, packets "hop" between routers. Packets are forwarded via a router's default path until it gets to destination IP. So is it possible to specify a specific set of routers this packet "hops" ...
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16 votes
2 answers
17k views

Why don't we use MAC address instead of IP for having "Internet" or doing communication? [duplicate]

We know MAC address is unique in each devices.So why can't we just send/receive data using MAC address only?
15 votes
2 answers
4k views

Why are there 3 ranges of private IPv4 addresses?

Why are there three separate ranges of private IPv4 addresses of different sizes? Why not just reserve the largest (10.0.0.0 - 10.255.255.255) range and let people create /16 or /24 (or whatever) ...
14 votes
3 answers
2k views

Where can I use the IPv6 documentation prefix

We are currently building a networking course with a practical part where students have to configure a small network in a virtual environment. However, we are not sure what ipv6 prefixes we can use ...
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14 votes
4 answers
1k views

Possible (bit) patterns of a netmask

Given a prefix Y, it's easy to calculate the corresponding netmask: Put Y times a set bit, and then fill up "to the right" with zeros until one has 32 bits (IPv4) in total. Example: Prefix 24, thus ...
14 votes
3 answers
32k views

How broadcasting works on different networks?

I was reading DHCP ( RFC 2131 ), I have basic knowledge about Relay Agent and Broadcasting. But for Understanding DHCP in depth, I felt like I have to know Broadcasting and Relay Agent ( RFC 1542 ) in ...
13 votes
4 answers
4k views

Difference between :: and ::1

I am confused about following netstat output. There are two kind of local ports. The first is: [::]8100 The second is: [::1]8100 I know that [::1] is a loopback address for IPv6. My question is: ...
13 votes
2 answers
7k views

Is the MTU of Ethernet still 1500 Bytes?

So basically do the modern routers in the internet still use 1500 as the MTU of Ethernet? or the 1500 Byte was for the old days? What I'm asking here is what is the MTU of Ethernet in modern Networks?...
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12 votes
3 answers
4k views

Why isn't multicasting more popular?

Multicasting seems provide an efficient method of routing networking traffic from a source to multiple end users, especially at this moment in time with teleconferencing, streaming media, online ...
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12 votes
3 answers
2k views

Are the IPv6 address space and IPv4 address space completely disjoint?

Tanenbaum's Computer Networks says Finally, IPv4 addresses can be written as a pair of colons and an old dotted decimal number, for example: ::192.31.20.46 Does it mean the IPv4 address space ...
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11 votes
3 answers
16k views

Is a TCP three-way handshake required for an HTTP POST?

I don't quite understand if i posting http form data from the browser to the server, does the protocol still need to make three-way handshake (syn-ack-data) or it only works for GET http requests?
11 votes
3 answers
7k views

Why do we need MAC Address if we can uniquely identify each machine with an IP Address

I learnt that in ARP Protocol, to send a packet from one machine to another machine we need to know the MAC Address of the destination machine but since the destination has already been assigned a ...
11 votes
3 answers
20k views

Find MAC address of a remote computer using IP

Is there a way to find out a computer's MAC address if we know its IP address, assuming this computer is outside our LAN? I know we can easily get the physical address of a computer in our LAN using ...
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10 votes
4 answers
3k views

Using IP subnet at multiple datacenters

We control IPv4 and IPv6 subnets. Now we'd like to use a part of a /24 IPv4 subnet at one datacenter and another part at another datacenter. I know that announcing the subnet on the internet at ...
10 votes
6 answers
8k views

Why do I need to specify a subnet mask on interface?

When assigning an IP address to an interface, why do I need to specify a subnet mask? I understand the purpose of subnet masks in a routing table for example (to specify a range of IP addresses as ...
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10 votes
1 answer
2k views

Does IP provide QoS or not?

IP provides a best-effort delivery service. I believe this implies, among other things, that it offers no QoS to the users. However, IP has a byte in its headers (ToS in v4 and Trafic Class in v6) ...
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10 votes
2 answers
895 views

ipv6 Aggregatable global unicast addresses

Studying IPv6 addressing I haven't understood IPv6 aggregable global unicast addresses, I mean on RFC 4291 there is a generic scheme: | n bits | m bits | 128-n-m bits ...
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9 votes
4 answers
3k views

Does every host on the LAN share the same ARP table, or do hosts keep them individually?

I'm just learning about ARP. I'm a little bit confused. My confusion is better explained in a scenario. Host A is communicating with Host B. Host A knows Host B's IP address, but not it's MAC address....
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9 votes
2 answers
2k views

Is there anything stopping me from using Class A addresses on my own network?

I've been reading about IPv4, networking, subnetting and the different classes of addresses. I understand that IP addresses beginning with numbers from 1 to 126 are considered "Class A" addresses. ...
9 votes
4 answers
3k views

What use are MAC addresses? [duplicate]

I know that MAC addresses are used in networks for several things, e.g. static fixed IP addresses assigned by DHCP servers or for Wireless access points. But why is it necessary in the Internet? Let'...
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9 votes
2 answers
4k views

What is IP squat space

I came across this article and I'm trying to understand what is going on here, I found some packets on my network containing 22.0.0.0/8 and 29.0.0.0/8 IP range and It's based on: https://en.wikipedia....
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9 votes
5 answers
11k views

Why is the protocol field part of an IP header?

Note this is not a duplicate of Why is the IP layer aware of higher layers in the network stack? The need for a protocol identifier (eg. the IP header's Protocol field) in packet-based communication ...
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9 votes
2 answers
2k views

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

According to RFC 4193, unique local addresses will always have a prefix of FD00::/8.. but according to Wikipedia: The block fd00::/8 is defined for /48 prefixes, formed by setting the forty least-...
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9 votes
6 answers
15k views

What is the purpose of subnetting, if we have NAT?

As I know, NAT is the system, that divides IP addresses into two categories - public and private. Public represents the local network from the outside and private represents the device from the inside....
9 votes
2 answers
11k views

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

From many tutorials, I get follow knowledge(maybe I'm misunderstood) : maximum of an Ethernet packet is about 1500 bytes. maximum of an IP packet is about 65535 bytes. maximum of a UDP packet is ...
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9 votes
1 answer
1k views

What happens when a subnet reaches capacity?

Other than not being able to claim any more IP addresses in a subnet, what other bad things happen when a subnet reaches 100% utilization, if anything?
9 votes
3 answers
3k views

How do VPN's forward network traffic? (Layer 3)

I am looking for some info on how VPN's (Virtual Private Network) forward network traffic through its VPS (Virtual Private Server). Take an example where you are connected to a VPN. You make a ...
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9 votes
1 answer
13k views

What is the use of the IPv6 scope ID?

In the structure struct sockaddr_in6, the field sin6_scope_id is declared. What is the use of this member? How can we know from the IPv6 address and the prefix length the scope that it should use?
9 votes
3 answers
5k views

Why do LANs use MAC addressing instead of IP addressing?

Why do LANs use MAC addressing instead of IP addressing? In other words, why can't a network just assign an IP address to a local area network device and send the data to that, instead of the MAC ...
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8 votes
2 answers
39k views

What exactly is IPoE?

I am familiar with PPPoE but getting confused with the term IPoE. Is this same as usual ethernet with DHCP server? Does this have any auth built-in? How do I get authenticated? What can a client ...
8 votes
4 answers
8k views

Usage of Flow label in IPv6 header?

I have been reading the IPv6 Flow Label Specification (RFC 6437) and I would like to ask about the purpose of setting this label. In case of just using the 2-tuple (source address, destination address)...
8 votes
4 answers
6k views

Why do I get "% Warning: use /31 mask on non point-to-point interface cautiously" when I use a /31 subnet mask?

Why do I get % Warning: use /31 mask on non point-to-point interface cautiously when I use a /31 subnet mask over Ethernet, and why am I not able to ping from R1 to R2 when both the interfaces have ...

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