Questions tagged [ipv4]

For questions about Internet Protocol version 4 (IPv4); as specified in RFC791.

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207
votes
6answers
397k views

The slash after an IP Address - CIDR Notation

What does the "/16" means in here : "192.168.0.0/16", for example.
113
votes
6answers
680k 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?
72
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 ...
61
votes
7answers
18k 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,...
45
votes
5answers
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 ...
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)?
39
votes
4answers
22k 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 ...
37
votes
8answers
29k 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 ...
37
votes
3answers
44k 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 ...
36
votes
13answers
58k 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 ...
33
votes
3answers
8k 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 ...
30
votes
9answers
77k views

Reason for both a MAC and an IP address

If my computer has an IP address,why does it need a MAC address?
24
votes
3answers
6k views

Will IPv6 make private IP addresses become obsolete?

Will it still be useful to make use of private IP addresses with IPv6?
24
votes
4answers
12k views

What is the proper term for ipaddress/hostname:port

What is the proper term for (example) hostname.tld:433 (hostname:portnumber)? It is not just hostname, and it is not really a URL either :) same goes for 10.0.0.1:3306 etc.
24
votes
1answer
782 views

Should IPv4 ICMP from untrusted interfaces be blocked?

Searching around I haven't been able to determine the best practice for ICMP on a firewall. For example on a Cisco ASA would it be safe and recommended to allow ICMP from any if ICMP inspection is ...
22
votes
3answers
9k views

Exactly when is PMTUD performed? (Path MTU discovery)

In discussions that have spurred from other questions on this site, I've realised that I don't have a solid understanding of when Path MTU Discovery (PMTUD) is performed. I know what it does -- ...
21
votes
7answers
7k 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 ...
20
votes
2answers
5k 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 ...
20
votes
4answers
22k views

Risks of using non-private IP addresses internally?

My company has received a large industrial machine with many networked devices on it. Unfortunately the engineer in charge has used a public IP address range on the machine. I'm in Europe. The chosen ...
20
votes
7answers
16k views

What is the purpose of network address? (aka subnet address)

What is the purpose of network address? (aka subnet address, starting address of each subnet) For example in this network: 192.168.1.0/24 Why we should leave 192.168.1.0 address free?
19
votes
1answer
5k 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.
18
votes
6answers
8k views

Why use IP if we already have a MAC address?

I am preparing for ICND1 exams and recently started to learn about different Cisco devices. I have just come to know how the packet is generated to be transmitted over a network, or outside the ...
18
votes
3answers
1k views

How does multihoming work with ipv6?

What are the differences between multihoming in IPv4 versus IPv6? Can an enterprise request a provider-independent IPv6 address space from its RIR/LIR which can be advertised to multiple upstream ...
17
votes
5answers
10k views

Usage of 192.168.xxx, 172.xxx and 10.xxx in private networks

I know that the organization that distributes IP addresses decided to assign 192.168.xxx, 172.xxx and 10.xxx to private networks. However, I thought that private networks have their own address space ...
17
votes
4answers
68k views

Confusing A, B, C network classes

I'm studying IPv4 addresses and came across this whole thing about classful addressing. I get the idea behind it, bit there is something I find confusing: There are two "ABC" ranges: First one: A: ...
16
votes
4answers
5k views

Why is 10.1.255.255 an invalid broadcast address?

I’ve been following the CCENT official certification book(100-105) and came upon this question in the “do I know this already?” quiz. The books only covered /24 subnetting only so far. Which of the ...
16
votes
4answers
49k views

Bad Mask for IP Address Error

I'm not too clear on subnetting rules and experienced an error when I tried to assign 148.26.1.176/28 to an interface on my Cisco router. Can someone please explain why it was not allowing me to ...
16
votes
3answers
11k views

mtr output with high packet loss on one hop

I'm investigating a complaint of poor performance from an end user accessing a site I help maintain. I have these two mtr outputs to the end user, the first from the site: ...
16
votes
2answers
7k views

IPv4 Address Space Planning Best Practices

A recent question from Craig Constantine pertained to IPv6, but many people are not on the leading edge with IPv6 yet and are still responsible for new or improved IPv4 deployments. I would like to ...
15
votes
6answers
25k views

How are IP addresses mapped to MAC addresses?

I've been reading about the differences between MAC and IP addresses, and why we need both of them. To summarize, MAC addresses are physical unchangeable unique IDs for every single device while IP ...
15
votes
2answers
3k 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
5answers
10k views

Duplicate IP Address - which one wins?

If there's a duplicate IP address, which one "wins"? First, last, flaps, neither?
14
votes
5answers
36k views

Finding IP Addresses for Social Network Sites

How do I find out a company like facebook's IP addresses. I am trying to block facebook at work and having some difficulties with HTTPs and URL blocking. Everytime I block a facebook IP more seem to ...
13
votes
4answers
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 ...
13
votes
3answers
24k 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 ...
13
votes
4answers
5k views

/31 Point-to-point bitmasks

When is it appropriate to use a /31 network in production, and is using them considered good practice? On a point-to-point link, broadcasts shouldn't be required, so is there any compelling case for ...
13
votes
2answers
9k views

What is a non-portable IPv4 address block?

For example, a Whois query for 4.4.4.0 says: ADDRESSES WITHIN THIS BLOCK ARE NON-PORTABLE Does the owner simply declare the block or a subnet are not up for sale? Or does non-portable have a ...
13
votes
3answers
2k views

VLAN and subnet relationship

Many have pointed out that a VLAN can hold one or more subnets. If your default gateway for this subnet is on a router or a L3 switch then how could there be more than one default gateway in a VLAN ...
13
votes
1answer
1k views

What happened to IPv5?

I hear/read much about IPv4 and IPv6 and nothing about IPv5. Where did it get lost in the discussion?
13
votes
1answer
3k views

Under what circumstances are multiple loopback (127/8) addresses useful?

I made an app in java where a client sends ip-packets to a server via the looback-address 127.0.0.1. A really simple app just to test how things work. At first I thought If I created several client-...
12
votes
4answers
9k views

What is the use of wild card masks vs normal masks

I've always wondered what the use of wild card masks was. When I learned about them in college I wondered what the use was, we saw that you could separate subnets by for instance even and uneven IPs. ...
12
votes
6answers
20k 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 (...
12
votes
3answers
16k 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 ...
12
votes
3answers
1k 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 ...
12
votes
4answers
2k views

How can I formulate the communication latency in TCP/IP?

I have difficulty in deriving a mathematical model/equation to estimate the round trip latency between two nodes communicating using TCP/IP. The nodes are exchanging data based on HTTP protocol. In ...
12
votes
3answers
23k views

Wifi TCP iperf throughput: 1 stream vs multiple streams?

In a WLAN iperf TCP throughput test, multiple parallel streams will give me higher throughput than 1 stream. I tried increasing the TCP window size, but I still cannot achieve the max throughput with ...
11
votes
3answers
1k views

Do today's routers prevent fake IP headers?

I know people can modify the IP headers and change source IP address, but it should be simple for network devices to detect those messages. If they don't, why is it so hard? Does it add too much ...
11
votes
3answers
2k views

How are the hosts in the tracert 216.81.59.173 able to use random domain names? [closed]

How are the host names in this trace route able to use random top level domains? I didn't know there was an 'Empire' or 'victory' top level domain. C:\Users\Chloe> tracert 216.81.59.173 Tracing ...
11
votes
2answers
15k views

Directed and Limited Broadcast confusion

Let's say I am on a network 192.168.1.X and I use the below ping commands. ping 255.255.255.255 ping 192.168.1.255 Why we are calling 1st one as limited and 2nd one as directed ? In what category ...
11
votes
2answers
3k views

Why don't iperf, scamper and path MTU discovery packet captures agree on the path's MTU?

Let's do some path MTU discovery between two Debian hosts separated by a Debian router that runs Shorewall-generated iptables rules. Each of the two hosts uses a single Ethernet link while the router ...

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