No evidence that this is bad for SEO and I think it's not.

No pagerank boost whatever you link anyway, this is old story and totally useless now.

I have no experience in hosting multiple websites on same host account. Will domain names have similar IP's?

Client is having real estate site - and I want to make individual sites for every broker and link them together.
Should I take single domain for every broker or multiple domain hosting account?

What would be better for SEO?

I suggest a top-level domain for the company and sub-domains for the brokers. This way all the broker pages can access some of the content (stats, listings, forms, maps, etc) on the main site and keep the PR in-house.

www.real-estate-site.com
> borker1.real-estate-site.com
> borker2.real-estate-site.com
> borker3.real-estate-site.com

You can have the hosting admin set it up, or change the pointers yourself.

Will domain names have similar IP's?

They will have identical IPs if they are all on the same server. That's where you get the whois lookups that say "83 websites share this address". Or "12" or "987" or whatever it may be.

If they will have identical IPs than I do not see benefit of hosting multiple domain names on one account. Correct me if I am wrong.

It must be bad for SEO as having multiple websites with similar content that share same IP - would be probably considered as spam.

I do not see benefit of hosting multiple domain names on one account.

Cost from supply/demand and ease of maintenance. There are a limited number of IP addresses, and your hosting company has an even more limited subset of those IPs.

If your hosting company limited 1 domain/IP, that would severely limit their abaility to make money. Even 1 IP/customer is beyond the ability of many hosting companies. Unles you have a dedicated server, it is likely that you will share the same IP with dozens or even hundreds of other domains.

So even if you were to create multiple hosting accounts, chances are they'd have the same IP. You'd have to specify that the accounts be located on different servers. But then you run into the obstacle of having to update your content across multiple servers.

In the end, Google knows about the IP address limit. It expects that many websites are going to share IPs. If it finds similar/duplicate content on different IPs, Google may figure out that you are trying to game the system, which may look more suspicious than having everything on the same IP address.

I have site with 1&1 linux shared hosting (business plan) – that is not dedicated IP – but we still have unique IP address that we do not share with others. We can type this IP in browsers and it gets to our site.

I am thinking of hosting other site with Hostgator linux shared hosting (Baby plan)
1. Will I have same unique IP as I have with 1&1?
2. If I add 3 domain names to that account – will they all have exactly the same IP? That means I can’t type IP in browser and get to each site?

I think the case of when it's "bad" is when you're going to "SEO" them all the same way. Totally unrelated sites all sitting on the same IP with the same source of links to them just looks bad.

I do not think it matters at all to have a unique IP address. On my shared hosting machine there are 80+ sites on 1 IP address and we all have different ranking in SERP, some doing extremely well. I sometimes see as many as 2k accounts on a server.

There are IPs with hundreds of thousands of domains on them. To correct my above post a little, if it's only your sites on that IP then it's more of an issue to pay attention to.

We can type this IP in browsers and it gets to our site.

That means you're missing a domain-name-canonicalization redirect. Having more than one way to reach the same domain is a much more serious problem than having multiple domains sharing an IP address.

I am thinking of hosting other site with Hostgator linux shared hosting (Baby plan)
1. Will I have same unique IP as I have with 1&1?
2. If I add 3 domain names to that account – will they all have exactly the same IP? That means I can’t type IP in browser and get to each site?

1. no.
ultimately the IP address is the location of a device, so that is unique.

2. in this case a request for the IP address would resolve to the default web server at that IP address, which might not even be one of the 3 domain names - it might be a hostname configured by the hosting provider or the IP address itself.