Knowledge base: IP addresses, DNS & SSL explained

What is an IP address?

An IP address (Internet Protocol address) is the unique number that identifies your device on the internet — much like a postal address for digital mail. Every website you visit sees this address, because without a sender the server would have nowhere to send its reply.

The address shown on our homepage is your public IP address: the address your internet connection presents to the outside world. You usually receive it from your internet provider, and all devices in your home share the same public address through your router.

IPv4 versus IPv6

IPv4 is the classic format: four numbers from 0 to 255 separated by dots, such as 203.0.113.42. Just over four billion IPv4 addresses exist — plenty in the 1980s, but long since exhausted. That is why providers share addresses and routers use techniques like NAT.

IPv6 is the successor, with addresses like 2001:db8::8a2e:370:7334. The number of possible addresses is so astronomically large that every device on earth could have billions. More and more providers ship IPv6 alongside IPv4 by default; our tool shows whether your connection supports both.

Public versus private IP addresses

Besides your public address, every device on your home network also has a private IP address, handed out by your router. These come from fixed ranges such as 192.168.x.x or 10.x.x.x and only work inside your own network — a website cannot do anything with them.

Your router constantly translates between inside and outside (NAT). That is how ten devices at home can share a single public address without traffic getting mixed up.

Why is the location not always correct?

The location attached to an IP address comes from databases that track which provider and region address ranges are assigned to. This tends to work reasonably well at country and region level, but the city can be far off — sometimes you will see your provider's office location instead of your home town.

It is therefore an estimate, not GPS. If you use a VPN, you will see the VPN server's location instead of your own — which is exactly what a VPN is for.

What is reverse DNS (PTR)?

Regular DNS translates a name into a number (proxxy.nl → IP address). Reverse DNS does the opposite: it looks up the name that belongs to an IP address, the so-called PTR record. Providers often name addresses something like x-x-x-x.provider.com. For mail servers a correct PTR record matters: mail from an address without proper reverse DNS is more likely to be flagged as spam.

What is a WebRTC leak?

WebRTC is the browser technology behind plugin-free video calls and screen sharing. To set up a direct connection, your browser exchanges IP addresses — and that can happen outside your VPN tunnel. A website could then see your real address while you believe you are anonymous behind a VPN: a WebRTC leak.

Modern browsers largely protect against this by masking local addresses with mDNS names ending in .local. Our test on the homepage shows what your browser reveals.

What are ports?

If your IP address is the postal address, ports are the letterboxes: numbered entrances (0 through 65535) behind which services listen. Websites use port 443 (HTTPS) and 80 (HTTP), secure terminal connections use port 22 (SSH), mail servers use port 25 (SMTP).

An open port means a service on your address is reachable from the internet. That is fine when intentional (say, your own server), but unintentionally open ports are a security risk. Our port check tests the most common ports on your own address.

What is DNS?

DNS (Domain Name System) translates the names people remember into the numbers computers use. Type proxxy.nl and DNS instantly looks up the matching IP address — just like you once looked up a name in a phone book to find the number. Without DNS you'd have to type strings of digits everywhere instead of names.

That translation happens through nameservers: specialised servers that hold a domain's data. Change something in your DNS (say, your site moves to a new server) and it takes a while for the whole world to know — that's the TTL (time to live), how long an answer may be cached.

The most important DNS records

A domain has several record types, each with its own job:

A and AAAA — point a domain to an IPv4 or IPv6 address (where is the website?).
MX — determines which mail servers receive email for the domain.
CNAME — lets one name point to another (for example www to the main domain).
TXT — free text, often used for email security like SPF and DMARC.
NS — indicates which nameservers are responsible for the domain.
CAA — records which authorities may issue an SSL certificate for the domain.

Tip: our DNS check shows all these records for a domain at once, plus a health check and the email settings.

DNS and email: SPF, DKIM and DMARC

A large share of spam and fraud consists of mail with a forged sender. Three DNS records help prevent this:

SPF defines which servers may send mail on behalf of your domain.
DKIM adds a digital signature to your mail, so the recipient can verify it wasn't altered in transit.
DMARC tells recipients what to do with mail that fails the checks — ignore, mark as spam, or reject.

Tip: if these aren't set correctly, your own mail lands in the spam folder more often. The email tab of our DNS check shows at a glance whether they're in order.

What is an SSL certificate?

An SSL certificate (really TLS these days) provides the padlock in the address bar and the https:// before a web address. It does two things: it encrypts the traffic between visitor and website (so no one can eavesdrop), and it proves the website really is who it claims to be.

Without a valid certificate, browsers show a "Not secure" warning and visitors quickly leave. For any serious website — especially with a contact or order form — a certificate is therefore essential.

Validity and renewal

Certificates are always valid for a limited time: often 90 days (with free Let's Encrypt) up to a year. If a certificate expires without being renewed in time, visitors immediately get that not-secure warning. Most hosting renews certificates automatically, but it stays wise to keep an eye on it.

Tip: our SSL check shows right away whether a certificate is valid, when it expires, who issued it and which domains it covers. Handy to check before things go wrong.

What to look for in a certificate

A few practical points:

Expiry date — renew well in advance, don't wait until the last day.
Covered domains — a certificate for example.com doesn't automatically cover www.example.com; check that both are included.
Issuer — a certificate from a recognised authority (like Let's Encrypt) is trusted by all browsers.
Wildcard — a certificate for *.example.com covers all subdomains at once.