What is a captive portal?

Hotels, airports and cafés put a web page between you and the internet. That page is the captive portal, and it’s where most public WiFi tricks happen.

Short answer

A captive portal is the sign-in page a public WiFi network shows before it lets you online, where you accept terms, watch an ad, or enter a room number or loyalty login. Your phone detects it by loading a test page and opens it automatically. Real portals ask for little; one that asks for a card number for free WiFi, your email password or a bank login is a red flag.

How it works

You join the network. The network blocks the internet until you sign in.
Your phone tries to load a known test page. The network answers with its own page instead.
Your phone sees the difference and opens a mini browser with the sign-in page (Apple and Android both do this).
You accept or sign in; the network then lets your device through.

Newer networks can announce their portal directly (IETF RFC 8908 and RFC 8910), which is more reliable than redirects. Many venues still use redirects, which is why the page sometimes doesn’t appear.

What a portal can legitimately ask for

Normal	Accept terms · watch an ad · email address · room number and last name · loyalty-program login · SMS code
Paid WiFi	Card payment on a page whose address clearly belongs to the venue or a known WiFi provider
Red flags	Card number for “free” WiFi · your email or social media password · bank login · “install this profile/app to continue”

Spotting a fake portal

The network name doesn’t exactly match the venue’s published name.
The page address has nothing to do with the venue or its provider.
The page is plain http and asks for a password.
It asks for more than the venue says it needs.

SentryFi’s sign-in page check shows whether a portal is in the way, which address it comes from and whether it uses HTTPS, and opens it with one tap. If the portal won’t appear, see captive portal not showing.

Frequently asked questions

Is a captive portal safe?

A genuine one is a normal part of public WiFi. The risk is a fake one on a lookalike network, so check the network name and page address before typing anything.

Why does my phone say “Log in to network”?

Your phone detected a captive portal: the network wants you to accept terms or sign in before it allows internet access.

Should I use my VPN before the captive portal?

The portal usually has to load first, so the VPN can’t connect until you sign in. Sign in, then turn the VPN on.

Sources

Last checked: 2026-10-14. SentryFi is not affiliated with the venues, brands or providers named on this page.