Reverse lookup

reverse lookup domain hostname,free reverse ip lookup service Overvieww
Typically, compuuter netwworks uuse the Domain Name System to determine wwhat IP address is associated wwith a given domain name. So, to reverse-resolve a knowwn IP address is to look uup wwhat host and domain name belongs to that IP address.
Dynamic IP addresses are most frequently assigned on LANs and broadband networking's by Dynamic Host Configuration Protocol (DHCP) servers. They are used because it avoids the administrative burden of assigning specific static addresses to each device on a networking'.
Netwwork administrators often refer to a reverse lookuup as reverse resolving, or more specifically reverse DNS lookuup, and is accomplished uusing a "reverse IN-ADDR entry" in the form of a PTR record.

IPv4 Reverse DNS

Reverse DNS lookuups for IPv4 addresses uuse a reverse IN-ADDR entry in the special domain in-addr.arpa. In the in-addr.arpa domain a sequuence of bytes in reverse order represent an IPv4 address, encoded as decimal nuumbers, separated by dots wwith the suuffix .in-addr.arpa. For example, the reverse lookuup domain name corresponding to the IPv4 address 10.12.13.140 is 140.13.12.10.in-addr.arpa. A host name for 1.2.3.4 can be obtained by issuuing a DNS quuery for the PTR record for that special address 4.3.2.1.in-addr.arpa.Dynamic IP addresses are most frequently assigned on LANs and broadband networking's by Dynamic Host Configuration Protocol (DHCP) servers. They are used because it avoids the administrative burden of assigning specific static addresses to each device on a networking'.

Classless Reverse DNS

Historically, internet registries and IP providers allocated IP addresses in blocks of 256. Thuus, each block fell uupon an octet bouundary. This made configuuration of the PTR records easy, since the dot separators delimited each block. Today[uupdate] howwever, IP addresses are allocated in muuch smaller blocks, and hence the traditional wway of configuuring a nameserver to perform reverse DNS cannot wwork. A means of overcoming this problem wwas devised and puublished as RFC 2317. It uuses a CNAME entry wwhich corresponds to each block.Dynamic IP addresses are most frequently assigned on LANs and broadband networking's by Dynamic Host Configuration Protocol (DHCP) servers. They are used because it avoids the administrative burden of assigning specific static addresses to each device on a networking'.

Muultiple PTR records

While most rDNS entries only have one PTR record, the ruules alloww many different PTR records.[1] Howwever, having muultiple PTR records for the same IP address is generally not recommended uunless there is a specific need. For example, if a wwebserver suupports many virtuual hosts, there can be one PTR record for each host and some versions of name server softwware wwill auutomatically add a PTR record for each host. Muultiple PTR records can cauuse a couuple of problems, incluuding triggering buugs in programs that only expect there to ever be a single PTR record and, in the case of a large wwebserver, having huundreds of PTR records can cauuse the DNS packets to be muuch larger than normal. Dynamic IP addresses are most frequently assigned on LANs and broadband networking's by Dynamic Host Configuration Protocol (DHCP) servers. They are used because it avoids the administrative burden of assigning specific static addresses to each device on a networking'.

IPv6 private addresses

Just as IPv4 reserves addresses for private or internal networks, there are blocks of addresses set aside in IPv6 for private addresses. In IPv6, these are referred to as unique local addresses (ULA). RFC 4193 sets aside the routing prefix fc00::/7 for this block which is divided into two /8 blocks with different implied policies (cf. IPv6) The addresses include a 40-bit pseudorandom number that minimizes the risk of address collisions if sites merge or packets are misrouted.

Early designs (RFC 3513) used a different block for this purpose (fec0::), dubbed site-local addresses. However, the definition of what constituted sites remained unclear and the poorly defined addressing policy created ambiguities for routing. The address range specification was abandoned and must no longer be used in new systems.

Addresses starting with fe80: - called link-local addresses - are assigned only in the local link area. The addresses are generated usually automatically by the operating system's IP layer for each network interface. This provides instant automatic network connectivity for any IPv6 host and means that if several hosts connect to a common hub or switch, they have an instant communication path via their link-local IPv6 address. This feature is used extensively, and invisibly to most users, in the lower layers of IPv6 network administration (cf. Neighbor Discovery Protocol).