What is IPv4?

Version 4 of IP. Its addresses are 32 bits, giving only 2^32 (about 4.3 billion), so they have long been conserved with private addresses and NAT. The header has a variable length of 20 to 60 bytes and holds the TTL, the number of the upper-layer protocol and a header checksum, among other fields. Routers along the path may also split (fragment) packets. Its successor with larger addresses is IPv6.

Japanese name: IPv4

IPv4 appears in 5 practice questions across 1 topic, including Computer Networking Basics.

Field: Computer Networking Basics

Try it: questions about IPv4

Which of the following IPv4 addresses falls within a private address range?

A172.32.0.1
B192.168.100.1
C11.1.1.1

Show answer and explanation

Answer: B. 192.168.100.1

The private address ranges defined in RFC 1918 are 10.0.0.0/8, 172.16.0.0/12 (172.16.0.0 to 172.31.255.255) and 192.168.0.0/16. They can be used freely inside organizations and homes, and because no routes for them are advertised on the internet, many organizations can reuse the same addresses. To communicate with the internet, a router's NAT translates them to global addresses. 172.32.0.1 has a second octet outside 16–31, so it is not in 172.16.0.0/12, and 11.1.1.1 is outside 10.0.0.0/8.

What is the role of the TTL (Time To Live) field in the IPv4 header?

AIt records the time from sending a packet until a response returns, to decide when TCP retransmits
BIt is decremented by 1 at each router, and packets that reach 0 are discarded, preventing packets from circulating forever in a routing loop
CPackets with larger values are forwarded with higher priority by routers

Show answer and explanation

Answer: B. It is decremented by 1 at each router, and packets that reach 0 are discarded, preventing packets from circulating forever in a routing loop

Despite "time" in its name, TTL is in practice an upper limit on the number of routers (hops) a packet may pass, decremented by 1 at each router. When it reaches 0, the router discards the packet and sends an ICMP time exceeded message back to the source. So even if a misconfiguration creates a routing loop, packets do not circulate forever. A packet sent with a TTL of 64, for example, arrives with 61 after passing three routers. traceroute sends packets with TTL 1, 2, 3 and so on, and discovers the path from the time exceeded messages each router returns. In IPv6 the field with the same role is called the hop limit.

IPv4 in the explanations

The 32-bit address routers use to choose routes is the IPv4 address, and what DHCP assigns is also an IP address.
Which statement about MAC addresses is correct?
An IPv6 address is 128 bits long (IPv4 is 32 bits), divided into eight 16-bit groups, each written as four hexadecimal digits.
Which correctly shortens the IPv6 address 2001:0db8:0000:00…